When solving the system of equations, x + 2y = 15 which expression could be substituted 5x + y = 21for x in the second equation?

c.

step-by-step explanation:

because first you have to add the 4x, 9x, and the 7x together and you get 20x and then you add the last numbers up and you get 5and then you put the subtraction sign in there and that's how you get the answer.