Since both equations are equal to y you can just make them equal to each other, like this 3x + 7 = x - 9. From there isolate the varible x on one side so that the solution will be on the other side of the equation. I'll do the steps too for you.
3x + 7 = x - 9
2x + 7 = -9
2x = -16
x = -8
Hope this helps :)
