Let's find the inflection points, we'll need to solve the equation for this
$$\frac{d^{2}}{d x^{2}} f{\left(x \right)} = 0$$
(the second derivative equals zero),
the roots of this equation will be the inflection points for the specified function graph:
$$\frac{d^{2}}{d x^{2}} f{\left(x \right)} = $$
the second derivative$$\frac{2 \left(\frac{2 \left(x + 3\right)^{2}}{x^{2} + 6 x + 8} + \frac{4 \left(x + 3\right)^{2}}{\left(x^{2} + 6 x + 8\right) \log{\left(x^{2} + 6 x + 8 \right)}} - 1\right) \log{\left(2 \right)}}{\left(x^{2} + 6 x + 8\right) \log{\left(x^{2} + 6 x + 8 \right)}^{2}} = 0$$
Solve this equationThe roots of this equation
$$x_{1} = 120168.633575834$$
$$x_{2} = -132714.274867548$$
$$x_{3} = -89202.6451737277$$
$$x_{4} = 85599.4150271294$$
$$x_{5} = -177587.29370865$$
$$x_{6} = -168523.568496299$$
$$x_{7} = -59902.1278157198$$
$$x_{8} = 205715.442806317$$
$$x_{9} = -72314.4303790954$$
$$x_{10} = 155696.481255537$$
$$x_{11} = -182134.217019303$$
$$x_{12} = -76502.8838889409$$
$$x_{13} = -64012.4958936911$$
$$x_{14} = -110756.78443908$$
$$x_{15} = -128294.077526429$$
$$x_{16} = 187391.063716$$
$$x_{17} = 169217.641089884$$
$$x_{18} = -55821.2210497517$$
$$x_{19} = 98430.7811721568$$
$$x_{20} = -150524.541787068$$
$$x_{21} = -123887.716463379$$
$$x_{22} = 128971.447821476$$
$$x_{23} = 72955.2642876424$$
$$x_{24} = 94134.4918327172$$
$$x_{25} = 196535.300613528$$
$$x_{26} = 137829.342273088$$
$$x_{27} = 210318.461242339$$
$$x_{28} = -195832.225532967$$
$$x_{29} = -191256.918749677$$
$$x_{30} = 191958.585932719$$
$$x_{31} = 77147.3877834874$$
$$x_{32} = 124562.914068797$$
$$x_{33} = 60530.5675385972$$
$$x_{34} = -93477.0672041229$$
$$x_{35} = -119495.687555167$$
$$x_{36} = 151211.752619206$$
$$x_{37} = 160192.585403383$$
$$x_{38} = 68787.4279361183$$
$$x_{39} = 142277.81044587$$
$$x_{40} = 164699.741069792$$
$$x_{41} = 52390.5864160638$$
$$x_{42} = 201120.989238219$$
$$x_{43} = 182832.962651317$$
$$x_{44} = -146053.375907282$$
$$x_{45} = -106411.08792188$$
$$x_{46} = -186690.839137426$$
$$x_{47} = -159501.845579111$$
$$x_{48} = 89857.0718999525$$
$$x_{49} = 81362.4971006734$$
$$x_{50} = 146738.741698596$$
$$x_{51} = -173050.322293489$$
$$x_{52} = -164007.31161835$$
$$x_{53} = -97770.4919774704$$
$$x_{54} = -68150.4777442292$$
$$x_{55} = 133393.743964823$$
$$x_{56} = -155007.480062937$$
$$x_{57} = -141594.347580223$$
$$x_{58} = 102745.118753583$$
$$x_{59} = 173745.993878637$$
$$x_{60} = -115118.52091079$$
$$x_{61} = 56444.9713258052$$
$$x_{62} = -137147.843704624$$
$$x_{63} = -80714.5160455582$$
$$x_{64} = -102082.087684706$$
$$x_{65} = 64645.3242495839$$
$$x_{66} = -84948.1315890447$$
$$x_{67} = 111424.967942615$$
$$x_{68} = 178284.522301674$$
$$x_{69} = 107076.747620113$$
$$x_{70} = 115789.130931816$$
You also need to calculate the limits of y '' for arguments seeking to indeterminate points of a function:
Points where there is an indetermination:
$$x_{1} = -4.41421356237309$$
$$x_{2} = -1.5857864376269$$
$$\lim_{x \to -4.41421356237309^-}\left(\frac{2 \left(\frac{2 \left(x + 3\right)^{2}}{x^{2} + 6 x + 8} + \frac{4 \left(x + 3\right)^{2}}{\left(x^{2} + 6 x + 8\right) \log{\left(x^{2} + 6 x + 8 \right)}} - 1\right) \log{\left(2 \right)}}{\left(x^{2} + 6 x + 8\right) \log{\left(x^{2} + 6 x + 8 \right)}^{2}}\right) = \infty$$
$$\lim_{x \to -4.41421356237309^+}\left(\frac{2 \left(\frac{2 \left(x + 3\right)^{2}}{x^{2} + 6 x + 8} + \frac{4 \left(x + 3\right)^{2}}{\left(x^{2} + 6 x + 8\right) \log{\left(x^{2} + 6 x + 8 \right)}} - 1\right) \log{\left(2 \right)}}{\left(x^{2} + 6 x + 8\right) \log{\left(x^{2} + 6 x + 8 \right)}^{2}}\right) = -\infty$$
- the limits are not equal, so
$$x_{1} = -4.41421356237309$$
- is an inflection point
$$\lim_{x \to -1.5857864376269^-}\left(\frac{2 \left(\frac{2 \left(x + 3\right)^{2}}{x^{2} + 6 x + 8} + \frac{4 \left(x + 3\right)^{2}}{\left(x^{2} + 6 x + 8\right) \log{\left(x^{2} + 6 x + 8 \right)}} - 1\right) \log{\left(2 \right)}}{\left(x^{2} + 6 x + 8\right) \log{\left(x^{2} + 6 x + 8 \right)}^{2}}\right) = 2.28359630832954 \cdot 10^{46} \log{\left(2 \right)}$$
$$\lim_{x \to -1.5857864376269^+}\left(\frac{2 \left(\frac{2 \left(x + 3\right)^{2}}{x^{2} + 6 x + 8} + \frac{4 \left(x + 3\right)^{2}}{\left(x^{2} + 6 x + 8\right) \log{\left(x^{2} + 6 x + 8 \right)}} - 1\right) \log{\left(2 \right)}}{\left(x^{2} + 6 x + 8\right) \log{\left(x^{2} + 6 x + 8 \right)}^{2}}\right) = 2.28359630832954 \cdot 10^{46} \log{\left(2 \right)}$$
- limits are equal, then skip the corresponding point
Сonvexity and concavity intervals:Let’s find the intervals where the function is convex or concave, for this look at the behaviour of the function at the inflection points:
Have no bends at the whole real axis