Answer
$x\sqrt[3]{3}$
Work Step by Step
Using the properties of radicals, the given expression, $
\dfrac{1}{3}\sqrt[3]{81x^3}
,$ simplifies to
\begin{array}{l}\require{cancel}
\dfrac{1}{3}\sqrt[3]{27x^3\cdot3}
\\\\=
\dfrac{1}{3}\sqrt[3]{(3x)^3\cdot3}
\\\\=
\dfrac{1}{3}\cdot3x\sqrt[3]{3}
\\\\=
\dfrac{1}{\cancel{3}}\cdot\cancel{3}x\sqrt[3]{3}
\\\\=
x\sqrt[3]{3}
.\end{array}
Note that all of the variables are assumed to represent non-negative real numbers.
