In order to solve the following system of equations by subtraction, which of the following could you do before subtracting the equations so that one variable will be eliminated when you subtract them? 4x-2y=7 3x-3y=15 a. multiply the top equation by -3 and the bottom equation by 2 b. multiply the top equation by 1/3 c. multiply the top equation by 15 and the bottom equation by 7 d. multiply the top equation by 3 and the bottom equation by 4