Answer
$\color{blue}{\bf{ 5280x }}$
$\color{blue}{\bf{\text{which is the number of feet in x miles }}}$
Work Step by Step
If $f(x)=3x$ is the number of feet in $x$ yards,
and $g(x)=1760x$ is the number of yards in $x$ miles:
$ ( f \text{ }\omicron\text{ g} )(x)=3(1760x)$
$\color{blue}{\bf{ 5280x }}$
$\color{blue}{\bf{\text{which is the number of feet in x miles }}}$