Answer:
the GCF of those numbers is 6.
Explanation:
One way to solve this is to list the factors of them, then find the greatest one that they share.
42: 1, 2, 3, 6, 7, 14, 21, 42
12: 1, 2, 3, 4, 6, 12
6: 1, 2, 3, 6
If we look at them, notice that six is in all of them. All of them have factors of six. Can we go any higher than that? No because six is the greatest factor of six, so we can't go any higher and still have all of them share a GCF. So your answer is 6.