hydrostatic force problem

I got this problem wrong and I need help because I don't know what I did wrong.

My answer: 2666.67 lb

Correct answer: 2133.3 lb

Here's the question:

A trough is filled with water and its vertical ends have the shape of the parabolic region in the figure. Find the hydrostatic force on one end of the trough.

http://i595.photobucket.com/albums/t...ceparabola.jpg

Here is my attempt:

When units are in feet, Force=δdA

Where δ=62.5 $\displaystyle lb/ft^3$

d= depth of the water

A= area

so I start of by finding the equation of the parabola by drawing axes on the picture.

http://i595.photobucket.com/albums/t...rabolaaxis.jpg

By looking at the graph I can tell the equation will look something like

$\displaystyle y=Cx^2+0$ //where C is a constant

using points on graph I get

$\displaystyle 4=C(4)^2$

4=16C

C=$\displaystyle \frac{1}{4}$ or 0.25

I get $\displaystyle f(x)=0.25x^2$

Next what I did was find the area

$\displaystyle A= \int_{-4}^4 0.25x^2 dx$

I integrated and i got

$\displaystyle A=\frac{32}{3} ft^2$

Now that I have found A i can plug all into the Force formula

F=δdA

=(62.5 $\displaystyle lb/ft^3$)(4ft)($\displaystyle \frac{32}{3} ft^2$)

F≈2666.67 lb <-------------- ANSWER IS INCORRECT PLEASE HELP!

What did I do incorrectly?