Whats the common factor of 18,27

Well, it depends, there may be multiple common factors, if it's the greatest common factor, then there is only one. The way to do this is to list all the factors of each number.

So Factors of

18: 1,2,3,6,9,18

27: 1,3,9,27

Common Factors : 1, 3, 9

Greatest Common Factor: 9