If f(x) =-e^(-2x-7)  and g(x) = 3/x , what is f'(g(x)) ?

$f ' \left(g \left(x\right)\right) = 2 {e}^{- \frac{6}{x} - 7}$
First thing is to calculate the derivate of $f \left(x\right)$,
$f ' \left(x\right) = 2 {e}^{- 2 x - 7}$
and now we substitute $g \left(x\right)$ in every $x$ of the $f ' \left(x\right)$,
f'(g(x))=2e^(-2·3/x-7)=2e^(-6/x-7)