One can use the Quotient Rule to differentiate functions of the form $\displaystyle f(x)=\frac{P(x)}{Q(x)}$. But, by rewriting $f(x)$ as $P(x)(Q(x))^{-1}$, one can use the Product Rule and the Chain Rule to differentiate $f(x)$. Try this approach to differentiate $\displaystyle f(x)=\frac{1}{3 x + 6}$.

$f'(x) =$