Answer
$3x^2\sqrt[4]{10x}$
Work Step by Step
Extracting the factors that are perfect roots of the given index, the given expression, $
\sqrt[4]{810x^9}
,$ simplifies to
\begin{array}{l}\require{cancel}
\sqrt[4]{81x^8\cdot10x}
\\\\=
\sqrt[4]{(3x^2)^4\cdot10x}
\\\\=
3x^2\sqrt[4]{10x}
\end{array}
* Note that it is assumed that no radicands were formed by raising negative numbers to even powers.