Answer:
6!
Explanation:
42 and 54 have a GCF of 6. To find the biggest common factor of 42 and 54, factor every digit (factors of 42 = 1, 2, 3, 6, 7, 14, 21, 42; factors of 54 = 1, 2, 3, 6, 9, 18, 27, 54) and select the greatest factor that divides both 42 and 54 exactly, i.e., 6.