The angles are marked as alternate interior angles. To be parallel, they must be equal. In other words x - 20 must equal 52. That's all you have to do. Just equate the 2 givens and solve.
x - 20 is one angle
52 is the other.
x - 20 = 52 Add 20 to both sides.
x - 20 + 20 = 52 +20 Combine
x = 72