9a3 - 15a2 + 3ab - 5b
So, after grouping, the expression would be something like this:
9a3 + 3ab - 5b - 15a2
=> 3a (3a2 + b) - 5(3a2 + b) (...taking the common factors out)
=> (3a2 + b) (3a - 5)
Remember, factoring is applicable to expressions containing at least 4 terms, with absolutely nothing in common.