1) Multiply top equation by 2 -> -10x-8y=30
2) Add both equations:
-10x-8y=30
+ 6x+8y=-18
__________
-4x=12
3) Divide both sides by -4 -> x=-3
4) Plug value of x into either equation:
6(-3)+8y=-18
-18+8y=-18
8y=0
y=0
So x=-3 and y=0
Hope this helped!