Hello there!
To start, a pair of supplementary angles are two angles that add up to a straight angle, or a total of 180 degrees.
Since you already know that one of the angles that supplement one another is 80 degrees, you can subtract 80 from 180 to get the measure of the angle that supplements 80 degrees.
As a result, you will get that 100 degrees is the measurement of the angle supplementary to 80 degrees. :)