the angles with parallel lines, are equal, this means:
and the angles A and D are congruent (opposite angles) and the same for C and B. So we can conclude, both angles we have are equal, so we can say:
7x-20=4x+16
and now we solve:
7x-4x=16+20
3x=36
x=36/3
x=12
So the answer is: x=12