I'm stuck on this derivative:

y=$\displaystyle \frac{x^2 + 2ax -a}{(x+a)^2}$

Here is what I have so far:

$\displaystyle \frac{(x^2+2ax+a^2)(2x+2a) - (x^2+2ax-a)(2x+2a)}{(x+a)^4}$

$\displaystyle \frac{2a^3+4a^2-2ax}{(x+a)^4}$

$\displaystyle \frac{2a(a^2+2a-x)}{(x+a)^4}$

I know I made a mistake somewhere but I can't find it??