Propostional logic help?

11. (P & R) → S, S ↔ T, ─T ├ R → ─P

12. P v Q, P → (T → S), P → T, S ↔ Q ├ S

13. A→(B→C), B & −C ├ −A

14. R & S, S → (P → Q), Q → P ├ P ↔ Q