You solve this by plugging one equation into the other. Usually you have to rewrite one equation to make this work. In this case I choose to rewrite y-4x=0 as y=4x.
After plugging it into the second, you get:
3x + 6*4x = 9 => 27x = 9 => x=1/3
Putting this solution back into y=4x gives us y=4/3
Complementary angles add to 90o
the other angle = x
the first angle = 3x - 4
x + 3x - 4 = 90
4x - 4 = 90
4x = 94
x = 94/4
x = 23.5
the first angle = 3*23.5 - 4 = 66.5
