The sum of the measures of angles in the polygon with n angles is
180°(n - 2)
We have an octagon → n = 8.
The sum of the measures of angles in an octagon is
180°(8 - 2) = 180°(6) = 1080°
The equation:
x + 139 + 140 + 125 + 116 + 152 + 2x + 143 = 1080
3x + 815 = 1080 subtract 815 from both sides
3x = 265 divide both sides by 3
x = 88 1/3
