Evaluate the following limit$.$ Use rH$$pital$\text{'}$s Rule when it is convenient and applicable.

$\underset{x-6}{lim}\frac{x^{2}9x18}{-48-2x{x}^2}$

$\underset{x-6}{lim}\frac{x^{2}9x18}{-48-2x{x}^2}=($Type an exact answer.$)$