Supplementary = 180 Degrees
So the measure of angle U + the measure of angle V = 180 Degrees
The measure of angle V is 3 times the measure of angle U, so
U+3U=180 Degrees, which can be written as 4U=180
You can find the value of U by dividing both sides by 4
U=180/4=45 Degrees
Since you are trying to find the value of x that makes U and V supplementary, you would sustitute the value of 45 Degrees that was calculated for U into the equation given for x.
45 Degrees = 3x
To solve you divide both sides by 3
x=45/3=15
x = 15 Degrees