Evaluate the following limits:
$\displaystyle\lim_{x\to - 3}\frac{x^2 - 9}{x^2 + 10 x + 21} =$

$\displaystyle\lim_{x\to -\infty}\frac{x^2 - 9}{x^2 + 10 x + 21} =$

$\displaystyle\lim_{x\to -6}\frac{\sqrt{3 x^2 - 72}- 6}{x + 6} =$