I'm working on this question:

if f(x) = x^3, then the derivative is 3x^2

I have to prove this using the definition of the derivative. I got as far as this:

$\displaystyle ((x^3)h + 2(h^2)(x^2)+(h^3)x-x^3)/(h)$

I'm unsure of how to simplify it in order to find the limit as h -> 0.