First, try multiplying multiple numbers to see if you get these numbers.
2=4,6,8,10,12,14,16,18,20,22,24,26,(28),30,32,34,36,38,40,(42)
7=7,14,21,(28),35,(42)
14= 14,(28),(42)
14, would be best; Alternatively, the GCF of 28 and 42 can be found using the prime factorization of 28 and 42.
-The factors of 28 are 28, 14, 7, 4, 2, 1.
-The factors of 42 are 42, 21, 14, 7, 6, 3, 2, 1.
Common numbers: 1 , 2, 7,14 , the largest number that can go into both is 14.