Ok, so you knnow that for angles to be supplement, they need to equal 180 degrees. So you can create an equation as follows:
(3 times X minus 12) degrees plus (x)degrees equals 180 degrees
(3x-12) + (x)=180
4x-12=180
4x=192
x=48
So now you plug in "x=48" into this equation (3x-12) to find the measure of the angle.
And if I did the math correctly, the answer should be "132" degrees.