In a trapezoid, two opposite sides are parallel and are called bases.
In this trapezoid, the top and bottom sides are parallel and are the bases.
The right side that intersects both bases is a transversal.
When parallel lines are cut by a transversal, same-side interior angles are supplementary. The measures of supplementary angles add to 180 deg.
x + 72 = 180
x = 180 - 72
x = 108 deg