Step-by-step explanation:
The sum of two interior angles in a triangle is equal to an exterior angle that's supplementary to the third interior angle
13x - 11 + 4x + 1 = 18x - 15 add like terms
17x - 10 = 18x - 15
- 10 + 15 = 18x - 17x
5 = x
measure of angle A 13x - 11 ➡ 13 × 5 - 11 = 54°
measure of angle B 4x + 1 ➡ 4 × 5 + 1 = 21°
measure of angle C 18x - 15 ➡ 18 × 5 - 15 = 75°
