f(x)=x^3
f(x+DELTA x) - x/ DELTA x
For some reason,
I can't figure it out.
It's not clear what your question is. Also please learn about order of operations and parentheses. I believe your second line is meant to convey
$\displaystyle \dfrac{f(x+\Delta x) - x}{\Delta x}$
Perhaps you missed an f and this should be
$\displaystyle \dfrac{f(x+\Delta x) - f(x)}{\Delta x}$
?
Note that $\displaystyle f(x+\Delta x) = (x+\Delta x)^3$. But if you want further help please ask a clear question.