Highest common factor of 72 and 63?

The factors of 72 are 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36, and 72

The factors of 63 are 1, 3, 7, 9, 21, and 63

The common factors of 72 and 63 are 1, 3, and 9