Use (n - 2) * 180 to find the sum of the measures of the interior angles of the polygon, where n = number of sides.
This polygon has 9 sides so plug in 9 for n.
(9 - 2) * 180 = 1,260
Divide this by the number of sides, 9.
1,260/9 = 140
Now divide this by 2 because x appears to be half of an interior angle (the radius is bisecting the interior angle).
140/2 = 70
x = 70°