Differentiating using first principles means they want you to use the limit definition of a derivative. There are two equivalent definitions that are used, which one did you learn?

Definition 1:

For a function , it's derivative, , at an arbitrary point is given by:

Defintion 2:

For a function , it's derivative, , at an arbitrary point is given by:

I think the first definition is what you want, so just plug the pieces into the formula and find the limit. (Do you need help with that?)