Find the common factor and factor out

9ab^2-3abc

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.