Need help with linear pairs!

I DON'T GET THIS.....

My book shows a picture of a linear pair and says to find the values of the the variables.

34.) shows the linear pair on one side it has y and on the other side it has nothing
then it shows the supplementry angles one side is (3x + 20) and the other side is (5x - 50)

PLEASE HELP!!!

A linear pair is a pair of angles formed by a straight line and a ray. What is the sum of the measures of the angles in a linear pair?

Eliz.