x = 30.
The bases of the trapezoid are parallel and the legs are congruent, which means the interior angles are congruent as well.
An equation can be set up where x+30 = 3x-30.
Add 30 to both sides (from right to left side); Subtract x from both sides (from left to right side).
60=2x
Divide both sides by 2 and:
30=x.
Here is a picture showing my work: