Question 127609

First of all you have a mistake
b^2/b^2-4 - 4/b^2-2b  =b^2/(b-2)(b+2) - 4/(b-2)(b)

the denominators are (b-2)(b+2) and (b-2)(b) to find the common denominator
you have to select (b-2) (because is in both denominators) and (b+2) and (b)

so the common denominator is (b-2)(b+2)b