Need help with derivative

Hello, this is my first post. I need help finding the derivative of $\displaystyle 1/x^2$ at x=2. The answer is supposed to be $\displaystyle -2/x^3$ and i need to find the derivative by definition. I have worked this out a million times and i keep making mistakes in my algebra i suppose because no matter what i try, i cannot get $\displaystyle -2/x^3$. any help would be greatly appreciated.

also, this is the first time ive ever heard of latex so hopefully i got it right, if i didnt i apologize