Since they are alternate interior angles, they will be congruent with each other.
4x= 5x-30
Subtract 5x on each side.
-1x= -30
Divide -1 into both sides.
x= 30
Now just plug in 30 too (4x) for the angle measure of both sides.
4*30 = 120
Each angle is equal to 120.
Hope this helps!