factors of 42 : 1, 2, 3, 6, 7, 14, 21, 42
factors of 216 : 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 27, 36, 54 ....
The rest of the factors of 216 do not matter, since 54 is already greater than 42, which is the largest factor of 42.
Hence, the largest common factor of the two numbers is 6.