Chapter 8 - Section 8.2 - Multiplying and Dividing Radicals - Exercise Set - Page 585: 34

$2x\sqrt{3}$

Getting the perfect factors and then extracting its root, the simplified form of the given expression, $ \sqrt{12x^2} ,$ is \begin{array}{l}\require{cancel} \sqrt{4x^2\cdot3} \\\\= \sqrt{(2x)^2\cdot3} \\\\= 2x\sqrt{3} .\end{array}