We just started differential equations in my AP Calculus class, and the only thing I am having trouble with is

. I tried using the binomial theorem (for cubing) but I get the wrong answer at the end. The only place I think I could be messing up is cubing

.

The problem:

Use the definition of the derivative to find f'(x).

f (x)=

My work thus far:

f' (x)=

+

DIVIDED BY

=

DIVIDED BY

=

DIVIDED BY

The

will cancel leaving:

I plugged 0 in for

and was left with

But I know this is the wrong answer (the right answer should be

)

**To clarify, DIVIDED BY

means everything on that line is divided by

. I can't figure out how to write

without the parentheses being around Delta. Nor can I figure out how to get

in the denominator of the whole expression. I tried though.