Solve the simultaneous equation 2x-4y=8 and 3x+3y=-15

Set 2x-4y=8 as equation 1, and 3x+3y=-15 as equation 2. Multiply equation 1 by 3 and equation 2 by 4 so they look like 6x-12y=24 and 12x+12y=-60. Now add the two equations and you get 18x=-36 or x=-2. Substitute this back into equation 2 and you get -6+3y=-15. Add 6 to both sides and you get 3y=-9 or y=-3.