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$$\frac{256 x \left(- \frac{6 x^{2}}{x^{2} - 16} + 3\right)}{\left(x^{2} - 16\right)^{3}} = 0$$
Solve this equationThe roots of this equation
$$x_{1} = 0$$
You also need to calculate the limits of y '' for arguments seeking to indeterminate points of a function:
Points where there is an indetermination:
$$x_{1} = -4$$
$$x_{2} = 4$$
$$\lim_{x \to -4^-}\left(\frac{256 x \left(- \frac{6 x^{2}}{x^{2} - 16} + 3\right)}{\left(x^{2} - 16\right)^{3}}\right) = \infty$$
$$\lim_{x \to -4^+}\left(\frac{256 x \left(- \frac{6 x^{2}}{x^{2} - 16} + 3\right)}{\left(x^{2} - 16\right)^{3}}\right) = \infty$$
- limits are equal, then skip the corresponding point
$$\lim_{x \to 4^-}\left(\frac{256 x \left(- \frac{6 x^{2}}{x^{2} - 16} + 3\right)}{\left(x^{2} - 16\right)^{3}}\right) = -\infty$$
$$\lim_{x \to 4^+}\left(\frac{256 x \left(- \frac{6 x^{2}}{x^{2} - 16} + 3\right)}{\left(x^{2} - 16\right)^{3}}\right) = -\infty$$
- 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:
Concave at the intervals
$$\left(-\infty, 0\right]$$
Convex at the intervals
$$\left[0, \infty\right)$$