The interior angles of any triangle always sum to 180°.
In the triangle, angles C and D are labeled congruent, so the missing angle has measure
180° - 2 (6x - 5)°
This angle is supplementary to the one labeled (11x + 1)°, which means
180° = (11x + 1)° + (180° - 2 (6x - 5)°)
Solve for x :
180° = (11x + 1)° + 180° - 2 (6x - 5)°
0 = (11x + 1)° - 2 (6x - 5)°
0 = (11x)° + 1° - (12x)° + 10°
(12x - 11x)° = 1° + 10°
x° = 11°
x = 11