Answer
$f(x)=x^{3}$, $a=5$
Work Step by Step
$f'(a)=\lim\limits_{h \to 0}\frac{f(a+h)-f(a)}{h}=\lim\limits_{h \to 0}\frac{(5+h)^{3}-125}{h}\,\,\,\,$ (given)
$\implies f(a+h)=(5+h)^{3}$ and $f(a)=125=5^{3}$
which gives $f(x)=x^{3}$ and $a=5$.