Answer
$$0$$
Work Step by Step
We know that $$ div F=\dfrac{\partial P}{\partial x}i+\dfrac{\partial Q}{\partial y}j $$
From the given equation, we have
$ div F=\dfrac{\partial}{\partial x}[0]+\dfrac{\partial}{\partial y} [0]+\dfrac{\partial}{\partial z} [(a^2-x^2-y^2)] \\ =0$
