P(x) => (Q(x) => R(x)) is equivalent to ~P(x) \/ ~Q(x) \/ R(x).

Existential quantifier distributes over disjunction (because existential quantifier is basically a disjunction over all elements of the domain): .

Next, existential quantifier changes into universal quantifier and vice versa when it is moved through a negation: .

These equivalences are sufficient to derive the one you need.