We know that two complements add up to 90.
Let's call the smaller angle x and the larger y.
3x = y
x + y = 90
We can use simple substitution.
x + 3x = 90
4x = 90
x = 22.5
Then, since we know that 3x=y, we can find the larger angle.
3*22.5 = 67.5
Y intercept slope - 5,0
X intercept slope - 0,-4
15 - x = 2(x + 3)
15 - x = 2x + 6
15 = 3x + 6
9 = 3x
3 = x
x = 3