Angle a & angle b are supplementary. the measure of angle b is 3x the measure of angle a. find angle a & angle b. how?

If the angles are supplementary, then you know that
they add up to 180 degrees.

A = 1 time A
B = 3 times A

Total:  (1 time A + 3 times A) = 4 times A

4 times A = 180 degrees

Divide each side by 4 :

1 time A = 180/4 = 45 degrees

A = 45 degrees

B = 3 times A = 135 degrees