Angles VSR and VSU are supplementary. This means there sum is 180 degrees. Knowing this you can make a formula:
angle VSR + angle VSU = 180 degrees
Lets make angle VSR x
x + 100 = 180
To solve this you must isolate x. To do this subtract 100 to both sides
x + 100 - 100 = 180 - 100
x = 80
The measure of angle VSR is 80 degrees
Hope this helped!
~Just a girl in love with Shawn Mendes