let me try
f(x) = x^3
[(x+h)^3 - x^3] / h
[x^3 + 3x^2h + 3xh^2 + h^3 - x^3] / h
[3x^2h + 3xh^2 + h^3] / h
[h(3x^2 + 3xh + h^2)] / h
3x^2 + 3xh + h^2
is this correct? sorry it look very ugly.
btw off topic; what software you use to type your math formula out, i guess it shouldn't be microsoft office or something since i don't know there's a format for that