Supplementary means that the angles add to 180. Thus, x + y = 180. But, we also have that x = 3y (stated in the question). Thus, we can substitute and say that 3y + y = 180 -> 4y = 180 -> y = 45. To find x, we substitute y into one of the two previous equations; I will use x = 3y. x = 3(45) -> x = 135.
