If you decompose 9 you will get 3*3 so 9=3*3
If you decompose 12 you will get 2*6,then you can decompose 6 which 2*3 so 12=2*2*3
If you decompose 6 you will get 2*3 so 6=2*3
What do all of these numbers have in common?The 3.
So 3 is the greatest common factor.Now you just put the 3 in front and then open the bracket and divide each of the number by 3.
9/3=3
-12x/3=4x
6y/3=2y
So,this will be your answer 3(3-4x+2y)