Two angles are complementary when they add up to 90 degrees
Two angles are supplementary when they add up to 180 degrees
angle = x
supplementary angle = (180-x)
complementary angle = (90-x)
180 - x - 40 = 2(90-x)
140 - x = 180 - 2x
-x + 2x = 180 - 140
x = 40°
