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$$\frac{3 \left(\sqrt{x} - \frac{3 \left(x^{2} + 2\right)^{2}}{4 x^{\frac{3}{2}} \left(x^{2} + 6\right)}\right)}{\sqrt{x^{2} + 6}} = 0$$
Solve this equationThe roots of this equation
$$x_{1} = - \sqrt{-6 + 4 \sqrt{3}}$$
$$x_{2} = \sqrt{-6 + 4 \sqrt{3}}$$
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
$$\left[\sqrt{-6 + 4 \sqrt{3}}, \infty\right)$$
Convex at the intervals
$$\left(-\infty, \sqrt{-6 + 4 \sqrt{3}}\right]$$