All the positive integers that will divide evenly into 18 are

1, 2, 3, 6, 9, and 18

All the positive integers that will divide evenly into 24 are

1, 2, 3, 4, 6, 8, 12, and 24

Now we look to see which positive integers are in both
lists.  Only these are in both lists:

1, 2, 3, and 6

Therefore they are the only factors which 18 and 24 have in common.

Edwin