# Thread: What do I do for this? (factoring binomials)

1. ## What do I do for this? (factoring binomials)

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.

2. Let $\color{red}u = a+b$ (just so it's easier to visualize).

So the problem becomes: $3a {\color{red}u} - {\color{red}u}$

Is this something you could factor ?

3. so I can't do anything to it?

4. Originally Posted by doomgaze
so I can't do anything to it?
please read o_O's post again. and think about what he said. he didn't put things in red just for fun. he is giving you a hint

5. Originally Posted by o_O
Let $\color{red}u = a+b$ (just so it's easier to visualize).

So the problem becomes: $3a {\color{red}u} - {\color{red}u}$

Is this something you could factor ?
so...

u(3a-1)

?

6. Exaactly. But what was ${\color{red}u}$ in the first place?

7. Originally Posted by o_O
Exaactly. But what was ${\color{red}u}$ in the first place?
(a+b) ?

so the answer to mine would be:

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

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

No, that's not right..

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

8. Originally Posted by doomgaze
(a+b) ?

so the answer to mine would be:

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

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

No, that's not right..

3a (a+b) - (a+b)
according to what you wrote before, which o_O confirmed, it is (a + b)(3a - 1). just replace u with a + b, that's it