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