This is a pentagon, a 5-sided polygon.
There is a formula to find the sum of the measures of the interior angles of a polygon of n sides. It is:
(n - 2)180
A pentagon has 5 sides, so the sum of the measures of the interior angles is
(5 - 2)180 = 3 * 180 = 540
The sum of the measures is 540.
We have every angle measure except the one marked x, but we know the sum all all 5 angle measures is 540.
100 + 102 + 108 + 121 + x = 540
431 + x = 540
Subtract 431 from both sides.
x = 109°