The expression ab-3a+4b-12 has to be factored.
= a(b - 3) + 4(b - 3)
= (a + 4)(b - 3)
The factored form of ab-3a+4b-12 = (a + 4)(b - 3)
ab-3a+4b-12 just find what they have in common
(b-3) (a+4) you can go even father if you want and get the main answer by setting the parenthesis equal to zero.
(b-3) = 0