Chapter 8 - Section 8.2 - Multiplying and Dividing Radicals - Exercise Set - Page 585: 22

$2\sqrt{10x}$

Getting the perfect factors and then extracting its root, the simplified form of the given expression, $ \sqrt{40x} ,$ is \begin{array}{l}\require{cancel} \sqrt{4\cdot10x} \\\\= \sqrt{(2)^2\cdot10x} \\\\= 2\sqrt{10x} .\end{array}