#### Answer

$(6x-1)\sqrt[]{x}$

#### Work Step by Step

Simplifying each term and then combining like terms, the given expression, $
2\sqrt[]{9x^3}-\sqrt[]{x}
,$ is equivalent to
\begin{array}{l}\require{cancel}
2\sqrt[]{9x^2\cdot x}-\sqrt[]{x}
\\\\=
2\sqrt[]{(3x)^2\cdot x}-\sqrt[]{x}
\\\\=
2\cdot3x\sqrt[]{x}-\sqrt[]{x}
\\\\=
6x\sqrt[]{x}-\sqrt[]{x}
\\\\=
(6x-1)\sqrt[]{x}
.\end{array}
* Note that it is assumed that all variables represent positive numbers.