To make an equation, make angle EOF = X and the angle that is supplementary to angle EOF = Y
An angle that is supplementary to another angle means that when you add the two angles, it equals 180 degrees.
So no matter what,
X + Y = 180
But the question asks us to find the angle that is supplementary to X, not what X plus the supplementary angle is equal to.
So to find that angle:
Y = 180 - X is the answer
