lets go step by step and factor out each as we go
first lets factor out a 3 since 3 is common in 9 and 3
3(3ab^2 - abc)
now lets factor out a since its common in both
3a(3b^2 - bc)
now lets factor out a b since its common in both
3ab(b - c)
we can't factor anything else out so we are done.
