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$$\frac{\pi^{2} \left(15957 \sin^{2}{\left(\frac{3 \pi x}{4} \right)} + 34880 \cos{\left(\frac{2 \pi x}{3} \right)} - 15957 \cos^{2}{\left(\frac{3 \pi x}{4} \right)}\right)}{72000} = 0$$
Solve this equationThe roots of this equation
$$x_{1} = -72.8737790478197$$
$$x_{2} = 30.5555182502932$$
$$x_{3} = 78.5555182502932$$
$$x_{4} = -312.87377904782$$
$$x_{5} = -357.975199088872$$
$$x_{6} = -53.4444817497068$$
$$x_{7} = -69.9751990888724$$
$$x_{8} = 33.9751990888724$$
$$x_{9} = -236.292165263802$$
$$x_{10} = -15.7078347361982$$
$$x_{11} = -17.4444817497068$$
$$x_{12} = 80.2921652638018$$
$$x_{13} = -389.444481749707$$
$$x_{14} = 20.2921652638018$$
$$x_{15} = 44.2921652638018$$
$$x_{16} = -93.9751990888724$$
$$x_{17} = -38.0248009111276$$
$$x_{18} = -62.0248009111276$$
$$x_{19} = -96.8737790478197$$
$$x_{20} = 59.1262209521803$$
$$x_{21} = -29.4444817497068$$
$$x_{22} = 45.9751990888724$$
$$x_{23} = -51.7078347361982$$
$$x_{24} = 57.9751990888724$$
$$x_{25} = -24.8737790478197$$
$$x_{26} = 11.1262209521803$$
$$x_{27} = -98.0248009111276$$
$$x_{28} = -74.0248009111276$$
$$x_{29} = -26.0248009111276$$
$$x_{30} = -279.707834736198$$
$$x_{31} = -14.0248009111276$$
$$x_{32} = -9.97519908887245$$
$$x_{33} = -81.9751990888724$$
$$x_{34} = -86.0248009111276$$
$$x_{35} = 92.2921652638018$$
$$x_{36} = -99.7078347361982$$
$$x_{37} = -41.4444817497068$$
$$x_{38} = -77.4444817497068$$
$$x_{39} = 98.0248009111276$$
$$x_{40} = 47.1262209521803$$
$$x_{41} = 2.02480091112755$$
$$x_{42} = 35.1262209521803$$
$$x_{43} = -60.8737790478197$$
$$x_{44} = -0.873779047819721$$
$$x_{45} = 66.5555182502932$$
$$x_{46} = 42.5555182502932$$
$$x_{47} = 54.5555182502932$$
$$x_{48} = 50.0248009111276$$
$$x_{49} = -57.9751990888724$$
$$x_{50} = -2.02480091112755$$
$$x_{51} = -84.8737790478197$$
$$x_{52} = -45.9751990888724$$
$$x_{53} = -50.0248009111276$$
$$x_{54} = -89.4444817497068$$
$$x_{55} = 23.1262209521803$$
$$x_{56} = 86.0248009111276$$
$$x_{57} = -3.70783473619817$$
$$x_{58} = 18.5555182502932$$
$$x_{59} = 9.97519908887245$$
$$x_{60} = -27.7078347361982$$
$$x_{61} = 95.1262209521803$$
$$x_{62} = -5.44448174970676$$
$$x_{63} = 26.0248009111276$$
$$x_{64} = -12.8737790478197$$
$$x_{65} = -33.9751990888724$$
$$x_{66} = -65.4444817497068$$
$$x_{67} = 90.5555182502932$$
$$x_{68} = 32.2921652638018$$
$$x_{69} = 81.9751990888724$$
$$x_{70} = -75.7078347361982$$
$$x_{71} = 21.9751990888724$$
$$x_{72} = -368.292165263802$$
$$x_{73} = 6.55551825029324$$
$$x_{74} = 14.0248009111276$$
$$x_{75} = -21.9751990888724$$
$$x_{76} = -329.444481749707$$
$$x_{77} = 83.1262209521803$$
$$x_{78} = 74.0248009111276$$
$$x_{79} = 62.0248009111276$$
$$x_{80} = 71.1262209521803$$
$$x_{81} = -87.7078347361982$$
$$x_{82} = -39.7078347361982$$
$$x_{83} = -36.8737790478197$$
$$x_{84} = 8.29216526380183$$
$$x_{85} = 68.2921652638018$$
$$x_{86} = 93.9751990888724$$
$$x_{87} = -63.7078347361982$$
$$x_{88} = 38.0248009111276$$
$$x_{89} = 56.2921652638018$$
$$x_{90} = -48.8737790478197$$
$$x_{91} = 69.9751990888724$$
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:
Concave at the intervals
$$\left[98.0248009111276, \infty\right)$$
Convex at the intervals
$$\left(-\infty, -357.975199088872\right]$$