Answer
$f(x)=5|(x-6)|$
Work Step by Step
Using $\sqrt[n]{x^n}=|x|$ if $n$ is even and $\sqrt[n]{x^n}=x$ if $n$ is odd, then
\begin{array}{l}\require{cancel}
f(x)=\sqrt{25(x-6)^2}
\\\\
f(x)=\sqrt{[5(x-6)]^2}
\\\\
f(x)=|5(x-6)|
\\\\
f(x)=|5|\cdot|(x-6)|
\\\\
f(x)=5|(x-6)|
.\end{array}
