Supplementary means that they add up to 180 degrees. Those 2 angles should add up to 180 degrees. X is the missing angle and 42 degrees is the other angle. If both are added together they equal 180. To find x we can simply subtract 42 from 180 to get the x. 138 is x. The proof is that if 138 and 42 are added together they end up making 180 degrees. Therefore, the answer is 138
