step-by-step explanation:
the ratio of red paint to blue paint is always 9/7. that means that, whatever x's value, (4x+12)/(4x-4) has that same ratio. so, set up an equation and cross-multiply to solve for x:
9/7 = (4x+12)/(4x-4)
9(4x-4) = 7(4x+12)
36x - 36 = 28x + 84
8x - 36 = 84
8x = 120
x = 15