#### Answer

$f$; $g(x)$

#### Work Step by Step

RECALL:
The composition function $f \circ g$ is defined as:
$(f \circ g)(x) = f(g(x))$
This means that all $x's$ in the function $f$ have to be replaced with $g(x)$
Thus, the missing expressions in the given statement (in order) are:
$f$; $g(x)$