Write the transformed equation for g(x) + 4, calling it h(x).

2x + y = x - y - 3 = 6
Add 3 to everything
2x + y + 3 = x - y - 3 + 3 = 6 + 3
2x + y + 3 = x - y = 9
2x + y + 3 = 9 
2x + y = 6 = x - y 
Add y to everything
2x + 2y = 6 + y = x
Since x = 6 + y substitute every x with 6 + y
2(6 + y) + 2y = x
12 + 4y = x

This is as simplified as I can get and then just place 12 + 4y into every x in the original problem then solve