Question: what is the greatest common factor of 42 and 63 using prime factorization?
Answer: The prime factorization of 63 is: 3 x 3 x 7. The prime factors and multiplicities 42 and 63 have in common are: 3 x 7. 3 x 7 is the gcf of 42 and 63. gcf(42,63) = 21.