Remember that two complimentary angles equal 90 degrees and supplementary, 180.
If a + b = 90, then a = 180 - b, right?
(180 - a) - 3(90 - a) = 10
180 - a - 270 + 3a = 10
2a = 100
a = 50
So, if an angle is 50 degrees, its complement is 40 and its supplement is 130.
130 - 3(40) = 10
130 - 120 = 10
10 = 10
Yep, 50 degrees.