Using natural deduction to prove that $\forall x \lnot (P(x) \lor R(x)) \implies \exists x(\lnot P(x) \lor \lnot R(x))$

Question

Not only do I not understand how to do this, but I don't comprehend the solution:

Here, supposons means assume, and donc means thus.

I'm specifically confused with line 5, for which I don't understand the rule $\implies E 2,3$ in the slightest; the lines it is referring to are not even implications! Any help would be appreciated, thanks!