Question

How do you combine `3/(b-3)-b/(b-3)`?

Answer 1

When you have the sum or subtraction of two fraction with the same denominator you simply mantain that denominator and sum/subtract the numerators:
`(3-b)/(b-3)=-(b-3)/(b-3)=-1`
I changed the sign by collecting `-1` on the numerator.

Answer 2

`3/(b - 3) - b/(b - 3) = (-(b - 3))/(b - 3) = -1`

Answer 3

When you have a sum or subtraction of fractions that share the same denominator, you can simply add/subtract the numerators, as follows:

`3/(b-3)-b/(b-3)=(3-b)/(b-3)`

Note that the numerator and the denominator are opposite, that is, one is the negative version of the other:

`(3-b)(-1)=(b-3)`, thus `(3-b)=-(b-3)`
and
`(b-3)(-1)=(3-b)`, thus `(b-3)=-(3-b)`

Thus, we can either rewrite it as

`((3-b)(-1))/(b-3)=(-(b-3))/(b-3)=-1`

or

`(3-b)/((b-3)(-1))=(3-b)/(-(3-b))=-1`

So, the final and shortest answer for your subtraction is `-1`