#### Answer

$3x$

#### Work Step by Step

Using the properties of radicals, the given expression, $
\sqrt[4]{81x^4}
,$ simplifies to
\begin{array}{l}
\sqrt[4]{(3x)^4}
\\\\=
3x
.\end{array}
Note that it is assumed that no radicand was formed by raising negative quantities to even powers. Thus, absolute value bars are not necessary.