Factor $24=2^{2}\times 6$. Rewrite the square root of the product $\sqrt{2^{2}\times 6}$ as the product of square roots $\sqrt{2^{2}}\sqrt{6}$. Take the square root of $2^{2}$.
$$-2\sqrt{6}-\frac{12}{\sqrt{6}}$$
Rationalize the denominator of $\frac{12}{\sqrt{6}}$ by multiplying numerator and denominator by $\sqrt{6}$.
