Pr(A n B) is the probability of both A and B happening (the n is an intersection sign). Pr(A|B) is the probability that A happens given that B happens: so P(A|B) = Pr(A n B) / Pr(B).

In your problem you're given Pr(B|A) which is Pr(B n A) / Pr(A). But Pr(A n B) = Pr(B n A): the event A and B happen is the same as the event B and A happen. You now have all the data required to find Pr(A n B) and Pr(A|B).

Incidentally Pr(A|B) and Pr(B|A) are quite different things. Think up a real-life example!