In a trapezoid, the angles are supplementary. Because the base lines are parallel, we know that the angles on the sides are same-side angles, which are supplementary.
Knowing this:
2x + (x - 12) = 180
(2x and (x - 12) are same side angles)
3x - 12 = 180
3x = 192
x = 64
Do the same for the other pair of same-side angles:
3y + (y + 20) = 180
4y + 20 = 180
4y = 160
y = 40
x = 64 and y = 40
Let me know if you need me to explain anything I did here :))
-T.B.
