Answer
$|x-4|$
Work Step by Step
We are told that $\sqrt[n] a^{n}=|a|$ when n is an even positive integer.
In this case, we must factor in order to find $a$.
$x^{2}-8x+16=(x-4)(x-4)=(x-4)^{2}$
Therefore, $a=(x-4)$.
$\sqrt (x-4)^{2}=|x-4|$