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