please help me to solve these

(ii) (~ P Q) in terms of only ( not and)

(b) Obtain Principal Disjunctive Normal and Principal Conjunctive Normal Form for the formula:
(P Q) (R S)

where P, Q, R and S logical (Boolean) Variables.

(c) Determine the validity of the conclusion S P from the following three premises:
~ P v Q , ~ (Q P v ~ R) and ~ (R ~ S) (R v ~ S)