The greatest common factor would be 6.
12 18
/ \ / \
6 2 6 3
/ \ / \
2 3 2 3
You put the greatest common factor outside of the parentheses
6(_+_)
Then on each blank within the parentheses you fill in what you would multiply by 6 to get the original numbers (12 and 18)
So, 6 (2 + 3)
