You are supposed to start with the definition of when a formula is true in an interpretation, i.e., the definition of the relation ⊧. You need to show that ∀x(A → P(x)) is logically equivalent to A → ∀xP(x). From left to right this means that for every interpretation I, if I ⊧ ∀x(A → P(x)), then I ⊧ A → ∀xP(x). So, assume I ⊧ ∀x(A → P(x)) and apply the definition of ⊧.