On their own, both of these statements are insufficient. To answer the question, we need the value of each of the two variables, but neither statement has enough information to find that.

Combined, they are sufficient. Add the two equations, and the result is 2x = 14, or x = 7. Plugging x = 7 into either equation, we find that y = 3 and thus, that xy = 21. Choice (C) is correct.