In order to find the extrema, we need to solve the equation
$$\frac{d}{d x} f{\left(x \right)} = 0$$
(the derivative equals zero),
and the roots of this equation are the extrema of this function:
$$\frac{d}{d x} f{\left(x \right)} = $$
the first derivative$$\frac{3 \cos{\left(\frac{3 x}{2} \right)}}{2} + 7 \tan^{2}{\left(7 x \right)} + 7 = 0$$
Solve this equationSolutions are not found,
function may have no extrema