(3) Algebra Problem: Can you help?

The question is:

3. Factorise 12ab – 18a2

and the answers are:

1. 3b(4b – 6a)
2. 6a(2b – 3a)
3. 2a(6a – 3b)

Can anyone help me please with how to GET to the answer?

So far I’ve tried finding the factors which multiply to make minus 18 but nothing I’ve found adds up to make positive twelve. Also the answer is arranged completely differently to the (x+yada)(x+yada) that i’m used to.

Thank you