Question 307555
Let the angle be A, then its complement is (90-A) and its supplement is (180-A).
(You have these two backwards!), so...
(180-A)-3(90-A) = 10 Simplify.
180-A-270+3A = 10 Combine the A's and the numbers.
(180-270)+(-A+3A) = 10 
-90+2A = 10 Add 90 to both sides.
2A = 100 Divide both sides by 2.
A = 50 degrees.