Supplementary means that the two angles add up to 180 degrees.
If mTUV= 75, and mMNO = (5x)
we can make an equation with them
75 + (5x) = 180
subtract 75 from both sides
(5x) = 105
now divide both sides by 5 to isolate the x
x = 21
now plug the value for x, 21, back into the original
This means that m MNO = 5(21)
do the multiplication
That equals 105 degrees.
Just to check plug it into the supplementary angle equation
180 ? 105 + 75
180 = 180
SO, mMNO equals 105 degrees.