Work Shown:
Let x and y be supplementary angles, which means they add to 180 degrees.
x = 72 is one of the angles. We don't know the other angle yet, so lets just keep it at y.
x+y = 180
72+y = 180 ..... plug in x = 72
72+y-72 = 180-72 ..... subtract 72 from both sides
y = 108
