We can change the sign of the second term, so that the two parenthesis are the same:
Now, both terms have in common a 3, becase the numeric factors are 3 and 6. If we factor the 3, we have
The parenthesis (a-1) is also in common, so we can factor it as well:
Finally, both terms in the parenthesis contain , because it's the power of b with the lowest exponent:
So, the factorization is
You can swap the order of the four factors (3, b^2, a-1 and 2b-1) as you like: the multiplication is commutative.