Verified answer

First note they all have a -3b in them so factorise that out first:

-3b(12a^2 - 7ab + b^2)

Then think of it like a regular trinomial.

What two numbers multiple to give 12 and add to give -7.

Obviously -3 and -4.

So it is:

-3b(12a^2 - 3ab - 4ab + b^2)

=-3b(3a(4a-b) - b(4a-b))

=-3b(3a-b)(4a-b)