Let x be the angle measure in degrees (now unknown). Then the supplement angle measure is (180-x) degrees. The condition says 180°-x = 3x - 10°. It is your basic equation. Simplify and solve it for x, step by step. 180 + 10 = 3x + x 190 = 4x x == 47.5. ANSWER. The angle is 47.5°. CHECK. The supplement angle is 180° - 47.5° = 132.5°. Thrice the angle is 3x = 3*47.5 = 142.5. 132.5° = 142.5° - 10°. ! Correct !