Chapter 9 - Roots and Radicals - 9.2 - Simplifying Radicals - Problem Set 9.2 - Page 406: 22

$-12 \sqrt{6}$

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 6 are factors of 54. We know that 9 is a perfect square, so we simplify: $-4 \sqrt{9} \sqrt{6}=-12 \sqrt{6}$