#### Answer

6 is the answer.
6 is given in both sets, 12 and 18

#### Work Step by Step

The factors of 12 are {1, 2, 3, 4, 6, 12}, and the factors of 18 are {1, 2, 3, 6, 9, 18}. The greatest common number in those sets is 6.
The Greatest Common Factor is the largest factor that is in both sets.