#### Answer

$3 \sqrt{14}$

#### Work Step by Step

In order to simplify a radical, we consider the factors of the number inside of the radical. If any of these factors are perfect squares, meaning that their square root is a whole number, then we can simplify the radical. We know that 9 and 14 are factors of 126. We know that 9 is a perfect square, so we simplify:
$ \sqrt{9} \sqrt{14}=3 \sqrt{14}$