The matching lines mean they have the same length so 12=5x-y and 4=3x+y. From here you can set up a system of equations as follows:
5x-y=12
3x+y=4
If you were to combine these two equations (imagine smooshing them) the y values would cancel out and the following equation would be left.
8x=16
Divide both sides by 8 and you get x=2. Plug this x value in and solve for y in either of the original equations.
5(2)-y=12
10-y=12
-y=2
y=-2