We are given:
Recall the difference of cubes formula:
Apply that to the numerator and something should cancel out Then you can plug in x = c without worrying about dividing by 0!
Don't forget the limit in front of each step!!! :
(You don't need the limit sign here since you already took the limit by replacing x with c)
And you can simplify the inside by collecting like terms:
So given any point c, the derivative at that point will be 15 times its square