Find f(a), f(a + h),
and the difference quotient
f(a + h) − f(a)
------------------
h
,
where h ≠ 0.
f(x) = 7x^3
f(a)=
f(a + h) =
f(a + h) − f(a)
----------------- =
h
That would be
(7(a+h)^3 - 7a^3)/h
Just expand the binomial, and the x^3 term will vanish, leaving evrything else with a power of h.
Divide by h. Done.