How do you simplify #(x^2 - x - 6) /( x^2 - 4)#?
We may not cancel in the numerator or denominator because there is not only one term. The + and - signs show there are 3 terms in the numerator an two terms in the denominator.
However, if we factorise we will only have one term and then we may cancel.
The numerator is a quadratic trinomial and the denominator is the difference between two squares.