This belongs in the Calculus section, not the elementary math section.

Ok. So using the definition of a derivative .

Thus for finding the derivative of we find that:

Multiply the numerator and denominator by

Split up the numerator to cancel some things:

And from here it's just algebra.

Distribute and apply the limit:

And once you get your answer, just plug in for

EDIT: For some reason, the Latex on the definition of the limit isn't working, but I think you're familiar with it.