The function $f$ is defined by $f(x)=\frac{4+x}{5+4 x}$.
Find $f(3 x)$
\[
f(3 x)=
\]
\(\boxed{f(3x) = \frac{3x + 4}{12x + 5}}\)
Step 1 :The function $f$ is defined by $f(x)=\frac{4+x}{5+4 x}$.
Step 2 :We are asked to find the value of the function $f(x)$ when $x$ is replaced by $3x$.
Step 3 :Substitute $3x$ into the function $f(x)$ to get $f(3x)$.
Step 4 :So, $f(3x) = \frac{4+3x}{5+12x}$.
Step 5 :\(\boxed{f(3x) = \frac{3x + 4}{12x + 5}}\)