what is the greatest common factor of 42 and 72

Question

and the answer  is out of these 2 3 9 6

Answer 1

42=2*3*7

72=2*2*2*3*3

what prime numbers do the two factorizations have in common ?

2 and 3

GCF=2*3

GCF=6

 

Answer 2

What is the greatest number in that list that can be used to divide both 42 and 72 and return a whole number?

 

42 can't be divided by 9 and return a whole number, so that's out.  Should be easy from there.