The factors of 12 are 1, 2, 3, 4, 6, and 12. As for 18, the factors are 1, 2, 3, 6, 9, and 18.
Hence, the common factors of 12 and 18 are 1, 2, 3, and 6.
If we see the GFC of 12 and 18, it's 6.