Each exterior angle measuring 2x forms a linear pair with an interior angle measuring an unknown measure. Angles in a linear pair are supplementary.
Since both interior angles are supplementary to congruent angles, they are congruent.
Let z be the measure of each upper interior angle.
2x + z = 180
z = 180 - 2x
Each upper interior angle measures 180 - 2x.
The sum of the measures of the interior angles of a triangle is 180.
180 - 2x + 180 - 2x + x = 180
-3x + 360 = 180
-3x = -180
x = 60
One interior angle measures x, so it measures 60 deg.
The other two interior angles measure 180 - 2x = 180 - 2(60) = 180 - 120 = 60
Answer: all interior angle measure 60 deg.
The exterior angles shown measure 120 deg each.