The derivative is correct. .
the thing is: I get +3 instead of -3.
perhaps so, but future readers of this thread may not click on, or view the image link you posted.
also, i'm no computational prodigy, but a bit more like a turtle: slow and sure wins the race. so to keep things straight in my mind, i have to verify. every. single. step (i discovered several computational errors before i finished my post). derivatives involving quotients are particularly nasty, because of the many opportunities for sign errors involved.