Carry out the binomial expansion in the numerator:
Then the 9⁴ terms cancel each other, so in the limit we have
Since <em>h</em> is approaching 0, that means <em>h</em> ≠ 0, so we can cancel the common factor of <em>h</em> in both numerator and denominator:
Then when <em>h</em> converges to 0, each remaining term containing <em>h</em> goes to 0, leaving you with
or choice C.
Alternatively, you can recognize the given limit as the derivative of <em>f(x)</em> at <em>x</em> = 9:
We have <em>f(x)</em> = <em>x</em> ⁴, so <em>f '(x)</em> = 4<em>x</em> ³, and evaluating this at <em>x</em> = 9 gives the same result, 2916.