To find the greatest common factor (GCF) of 18 and 42, we can list the factors of each number.
The factors of 18 are: 1, 2, 3, 6, 9, 18
The factors of 42 are: 1, 2, 3, 6, 7, 14, 21, 42
Now, we identify the common factors: 1, 2, 3, 6
The greatest common factor is the largest of these common factors, which is 6.
Therefore, the GCF of 18 and 42 is 6.