Answer
$f\left( -x \right)=3x+4$
Work Step by Step
Substituting $x$ with $
-x
$ in the given function, $
f(x)=-3x+4
,$ then,
\begin{array}{l}\require{cancel}
f\left( -x \right)=-3\left( -x \right)+4
\\\\
f\left( -x \right)=3x+4
.\end{array}