actually nevermind, I figured out where i went wrong.
x^2 + xy = 5,
I figured out that f'(x)= (-2x-y)/x, and that f''(x)= (y-xy')/x^2.
I plug-in f'(x) into f''(x) and I get, y-x(-2x-y)/x^3, and I simplified it to (y+2x^2+xy)/x^3. rewriting it as (y+2(5))/x^3, then (y+10)/x^3
The back of the book says it is supposed to be just 10/x^3, where is this extra y coming from?