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.