How do you factor polynomial by factoring out the oppisite of gcf

here is a problem

-18 a^2b + 12ab^2

the example in my math book was confusing and want to see if someone can give me a better example

I look at each part individually, so for your problem we have -18, a^2, b, 12, a, and b^2.
Between -18 and 12 the gcf is 6.
Between a^2 and a the gcf is a.
Between b and b^2 the gcf is b.
So in total we need to pull out 6ab, and we have
6ab(-3 a^2b + 2ab^2)
To make the inside start with a positive number, we pull out a -1 to get
-6ab(3a^2b-2ab^2).