How do you combine #3/(b-3)-b/(b-3)#?
When you have the sum or subtraction of two fraction with the same denominator you simply mantain that denominator and sum/subtract the numerators:
I changed the sign by collecting
When you have a sum or subtraction of fractions that share the same denominator, you can simply add/subtract the numerators, as follows:
Note that the numerator and the denominator are opposite, that is, one is the negative version of the other:
Thus, we can either rewrite it as
So, the final and shortest answer for your subtraction is