could someone help me understand this ?

Question

Factor the following by taking out the greatest common factor.

-12a²b+42ab²+36a²b²

 

im so confused i know that the common factor is 6. but i dont know what to do from here. 

Answer 1

What factors do all the terms have in common?  They have a factor of 6 in common.  They have a factor of a in common, and they have a factor of b in common.  So the greatest common factor is

 

6ab

 

Can you figure out the rest?