Help with simplifying integral

[xnitexlitex]

Homework Statement

We need to solve the double integral:

6 -2/3x+4
∫ ∫ [12-2x-(3y/4)] dy dx
0 0

Homework Equations

(a+b)² = a² + 2ab + b²

The Attempt at a Solution

I've got to:

6
(1/4) ∫ [12y-2xy-(3y²/2)] evaluated at -2/3x + 4 and 0.
0
6
(1/4) ∫ [12(-2/3x + 4) - 2x(-2/3x + 4) - (3(-2/3x + 4)²/2)]
0

This is where I'm stuck. I could solve the rest of the integral if I could simplify this right. Sometimes, I think simplifying the integral is harder than the integration itself.

 

[ArcanaNoir]

6
(1/4) ∫ [12y-2xy-(3y2/2)] evaluated at -2/3x + 4 and 0.
0
6

This isn't right.Where did the 1/4 come from? And it's not (3y2/2), it's 3y²/8.
Which, (1/4)(3/2)=(3/8), but then the other terms are wrong, because (1/4)12= 3, and it's not 3y.