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$$\frac{2 \left(- \frac{\left(1 - \frac{3}{x^{2}}\right)^{2}}{x + \frac{3}{x}} + \frac{6}{x^{3}}\right)}{x + \frac{3}{x}} = 0$$
Solve this equationThe roots of this equation
$$x_{1} = - \sqrt{6 + 3 \sqrt{5}}$$
$$x_{2} = \sqrt{6 + 3 \sqrt{5}}$$
You also need to calculate the limits of y '' for arguments seeking to indeterminate points of a function:
Points where there is an indetermination:
$$x_{1} = 0$$
$$\lim_{x \to 0^-}\left(\frac{2 \left(- \frac{\left(1 - \frac{3}{x^{2}}\right)^{2}}{x + \frac{3}{x}} + \frac{6}{x^{3}}\right)}{x + \frac{3}{x}}\right) = \infty$$
Let's take the limit$$\lim_{x \to 0^+}\left(\frac{2 \left(- \frac{\left(1 - \frac{3}{x^{2}}\right)^{2}}{x + \frac{3}{x}} + \frac{6}{x^{3}}\right)}{x + \frac{3}{x}}\right) = \infty$$
Let's take the limit- 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
$$\left[- \sqrt{6 + 3 \sqrt{5}}, \sqrt{6 + 3 \sqrt{5}}\right]$$
Convex at the intervals
$$\left(-\infty, - \sqrt{6 + 3 \sqrt{5}}\right] \cup \left[\sqrt{6 + 3 \sqrt{5}}, \infty\right)$$