Answer
$f(x)=9|x-1|$
Work Step by Step
Using the properties of radicals, the given equation, $
f(x)=\sqrt[]{81(x-1)^2}
,$ simplifies to
\begin{array}{l}\require{cancel}
f(x)=\sqrt[]{\left[ 9(x-1) \right]^2}
\\\\
f(x)=|9(x-1)|
\\\\
f(x)=9|x-1|
\end{array}
