so I was doing my homework today and I stumbled upon this problem, I had no trouble with any of the other ones and I look at the problem and I have no idea what to do.

problem:

factor problem using the greatest common binomial factor.

3a(a+b)-(a+b)

so is it just.

(3a)(-1)(a+b)

or

3a(a-b)^2

haha, I'm sure this problem is really easy, I don't know why I have no idea how to do it.