The answer to the following question is C = -3.75. Please tell me where I went wrong:

Question: y = integral (squareroot 3x - cuberoot 9x+3x)dx, where x = 3 and y =9

Let U = 9x + 3x

du = 9 + 3 dx

du = 12 dx

y = integral [(3x)^2] - integral (U)^3

y = [(3x^3)/3] - [(U^4)/4] + C

y = x^3 - [(9x+3x)^4]/4 + C

9 = 3^3 - [(9{3}+3{3})^4]/4 + C

9 = 27 - [(27+9)^4]/4 + C

9 = 27 - 1679616/4 + C

C = 419886 (According to my teacher, the answer is C = -3.75)