If you have a fraction, then the denominator (bottom) cannot equal 0, since it is impossible to divide by 0. Take a look at the function below.
[tex]y=\frac{5}{x-2}[/tex]
If we plugged in 2 for x, we would end up with a denominator of 0, making x = 2 "extraneous."
(you can use this example to answer your question, with the denominator being x-2...any expression that evaluates to 0 when x = 2 would work)
