Complex sentence: The hardness proof is trivial, as S5 includes the propositional logic; the membership part is proved by showing that any satisfiable formula has a Kripke model where the number of worlds is linear in the size of the formula.
Split into simpler sentences: The hardness proof is trivial, as S5 includes the propositional logic. Membership is proved by showing that any satisfiable formula has a Kripke model, where the number of worlds is linear in the size of the formula.