Angle 1 = A
Angle 2 = B
They are supplementary, that means that they add 180 degrees, then: A + B = 180
One angle measures 12 degrees less than 3 times the other, tahn means: A - 12 = 3B
Equation 1: A + B = 180
Equation 2: A - 12 = 3B
Solving for A in equation 1:
A + B = 180
A = 180 - B
Using this value into equation 2, and solving for B:
A - 12 = 3B
(180 - B) - 12 = 3B
180 - B - 12 = 3B
168 = 3B + B = 4B
4B = 168
B = 168/4 = 42
B = 42
Using the expression we found for A:
A = 180 - B = 180 - 42 = 138
A = 138
Answer
42 degrees and 138 degrees