Change the fraction w-3/w+5 into an equivalent fraction with the denominator w^2+w-20

In order to get the denominator from (w+5) to (w^2+w-20) you have to multiply the (w+5) by (w-4), but you can't just multiply the denominator. In order to make the fractions equivalent anything you do to the denominator you must do to the numerator. Therefore the answer would be [(w-3)(w-4)]/[(w+5)(w-4)] which would simplify to (w^2-7w+12)/(w^2+w-20)