Answer
$y=f[g(x)],$
$f(x)=-\sqrt{x},\qquad g(x)=13+7x$
Work Step by Step
A reliable method is asking:
"How would we calculate this on a calculator?".
First, calculate $g(x)=13+7x$
and then, calculate: $\quad -\sqrt{\text{the result, g(x)}}$
(apply $f(x)=-\sqrt{x}$ on g(x) instead of x)
$f[g(x)]=-\sqrt{g(x}=-\sqrt{13+7x}=y$
