The correct answer is C

Explanation

Recall that x^2 minus y^2 can be factored as (x + y) times (x minus y). Since x + y = 3 and x minus y = 5, their product, x^2 minus y^2, is (3) times (5), or 15.

Another method of solution is to solve the two equations x + y = 3 and x minus y = 5 simultaneously to get x = 4 and y = negative 1. Then x^2 minus y^2 is 4^2 minus (negative 1)^2 = 16 minus 1 = 15.