Simplify each expression.
$\frac{x-7}{7-x}$

Simplify the rational expressions below:

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

Simplify the rational expressions below:

$\frac{x^2-9}{x^2-3x}$

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.

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

Simplify each expression.

$\frac{x^2-10x+24}{(4-x)(6-x)}$

For which of the following values of $x$ is the rational expression undefined?

$\frac{5}{x}$

For which of the following values of $x$ is the rational expression undefined?

$\frac{7}{x-3}$