#### Answer

$y=f[g(x)],$
$f(x)=\sqrt{x},\qquad g(x)=9-4x$

#### Work Step by Step

A reliable method is asking:
"How would we calculate this on a calculator?".
First, calculate $g(x)=9-4x$
and then, calculate: $\quad \sqrt{\text{the result, g(x)}}$
(apply $f(x)=\sqrt{x}$ on g(x) instead of x)
$f[g(x)]=\sqrt{g(x)}=\sqrt{9-4x}=y$