Currently struggling with a lecture problem, any help with be greatly appreciated

(15 points) Let X be a random variable with sample space 2. Suppose that Prob is a probability function on P(), the power set of 2. Let BE P() be a subset of 2 such that Prob(B) > 0. For any A E P(), define the function Prob( A| B) = Prob(An B) Prob(B) Prove that Prob(A B), as a function of A, is a probability function