We are given an image shown below:
I
Given the image above, since angle 80 degrees and angle y are opposite by the vertex, they have the SAME measure.
so y = 80 degrees
and since x is the supplementary angle to y, than their addition must be 180 degrees. Then, we can write and solve the following equation:
x + y = 180 dgrees
x + 80 = 180 degrees
then x = 180 - 80 = 100 degrees
x = 100 degrees.
