What is the solution to this system of equations?

x − 2y = 15
2x + 4y = -18

Use the subtraction method.
 2 (x - 2y = 15)      multiply top equation by 2 so we can subtract

2x - 4y = 30
2x + 4y = -18   (subtracted, we get 4x = 12...divide by 4...x = 3)

place the value for X into an original equation and solve for Y.

2(3) + 4y = -18
6 + 4y = -18
-6            -6
4y = -24  (divide by 4)

y = -6      Solution(3, -6)