Question 1000816
"... and the binomial theorem."  You mostly need to expand that part in the difference quotient, specifically this part of the expression:


{{{f(x+h)=(x+h)^3}}} and refer directly to the binomial theorem formula to be really formal about this.  To be simpler about this,  {{{(x+h)^3=x^3+3*x^2*h+3x*h^2+h^3}}}.