Factor ab-3b+b^2-3a

$a b - 3 b + b^{2} - 3 a$

Regroup terms.

$b^{2} + a b - 3 b - 3 a$

Factor out the greatest common factor from each group.

Group the first two terms and the last two terms.

$(b^{2} + a b) - 3 b - 3 a$

Factor out the greatest common factor (GCF) from each group.

$b (b + a) - 3 (b + a)$

Factor the polynomial by factoring out the greatest common factor, $b + a$ .

$(b + a) (b - 3)$