Proceed as follows:
(a) Copy the wff, and underline the free occurrences of variables in it.
(b) List the free variables of the wff.
(c) List the bound variables of the wff.
(d) Decide whether $g x$ is free for $u$ in the wff.
(e) Decide whether $g x$ is free for $z$ in the wff.
(f) Decide whether $h x y$ is free for $u$ in the wff.
(g) Decide whether $u$ is free for $x$ in the wff.
$\sim[[\forall u P u z \wedge \sim \exists x=Q u x \vee \forall z \sim R x z] \supset \exists z Q u z]$