GCF means "greatest common factor."
When I learned this, I was told to try 5 numbers and see if they work and if not, it might be 1.
Those numbers are 2, 3, 5, 7, and 9.
2 goes into 56 but not 21.
3 goes into 21 but not 56.
5 doesn't go into 56 nor 21.
7 goes into both 56 and 21.
9 doesn't go into 56 nor 21.
So since 7 works with both numbers, try numbers divisible by 7 (i.e. 14, 21...)
14 goes into 56 but not 21.
21 goes into 21 but not 56.
You can't go any higher than that since then it would be going above 21.
So the GCF of 56 and 21 is 7.