[tex] \bf -7\sqrt{108}~~\begin{cases}108=2\cdot 2\cdot 3\cdot 3\cdot 3\\\qquad 2^2\cdot 3\cdot 3^2\\\qquad (2^2\cdot 3^2)\cdot 3\\\qquad (6)^2\cdot 3\end{cases}\implies -7\sqrt{6^2\cdot 3}\\\\\\-7\cdot 6\sqrt{3}\implies -42\sqrt{3} [/tex]
