This is a good one!
So we know that a triangle's angles must add up to 180 degrees. Therefore, we can say that:
(3x - 3) + (5x + 3) + (4x) = 180.
This can be rewritten as 12x = 180.
Divide each side by 12 to get x alone, you get:
x = 15
Time to plug n' chug.
4x = 60, 3x - 3 = 42, 5x + 3 = 78
Boom!