What is the GCF of 42 and 126?

The factors of 42 are:

1, 2, 3, 6, 7, 14, 21, 42

The factors of 126 are:

1, 2, 3, 6, 7, 9, 14, 18, 21, 42, 63, 126

The common factors are:

1, 2, 3, 6, 7, 14, 21, 42

The Greatest Common Factor (GCF) is:

42