The sum of the measures of the interior angles of an n-sided polygon is 180(n - 2).
A pentagon has 5 sides, so for a pentagon, the sum of the measures of the interior angles is
180(n - 2) =
= 180 (5 - 2)
= 180 * 3
= 540
Since this pentagon is regular, all interior angles are congruent.
Each one measures 540/5 = 108, or 108 degrees.
The upper 108-degree angle together with the 2 angles marked x form a straight angle.
x + x + 108 = 180
2x + 108 = 180
2x = 72
x = 36
