Chapter 8 - Section 8.2 - Multiplying and Dividing Radicals - Exercise Set - Page 585: 23

$3\sqrt{x}$

Getting the perfect factors and then extracting its root, the simplified form of the given expression, $ \sqrt{9x} ,$ is \begin{array}{l}\require{cancel} \sqrt{9\cdot x} \\\\= \sqrt{(3)^2\cdot x} \\\\= 3\sqrt{x} .\end{array}