Simplify the rational expressions below:
$\frac{x^2-9}{x^2-3x}$

Evaluate the rational expression below at $x=-1$.

$\frac{12}{x+3}$

Evaluate the rational expression below at $x=2$.

$\frac{x^2-4}{x^2-x-6}$

Evaluate the rational expression below at $x=-2$.

$\frac{3x}{x^2+x-6}$

Simplify the rational expressions below:

$\frac{6x}{12x}$

Simplify the rational expressions below:

$\frac{x^2-4x}{x^2-2x-8x}$

Simplify the rational expressions below:

$\frac{x^2+5x+6}{x^2+7x+10}$

Simplify each expression.

$\frac{x-7}{7-x}$

Simplify each expression.

$\frac{x^2-16}{16-x^2}$