why am I getting "x" in the denominator?

Hi

I was solving an example problem using a different method using a formula linked below. The answer I ended up has an "x" in the denominator which isn't there in solution the book presents. I've failed to figure out where I made a mistake. Could you please help me? Thanks a lot.

Book's solution: https://img810.imageshack.us/img810/8342/examplek.jpg
My Solution: https://img717.imageshack.us/img717/2970/img0003wu.jpg
Formula used: https://img411.imageshack.us/img411/8596/exactequation.jpg

With best regards
PG

The algebraic expression under the integral in line 7 should reduce to just 2y. The integral of that is y^2, which simplifies the remaining lines to the correct answer.