Respuesta :

irspow
You need to find the common primes...

18=2*3*3  and 72=2*2*2*3*3  so they have 2*3*3 in common and 

2*3*3=18

So 18 is the greatest common factor.
Greatest Common Factor(GCF): Largest factor of two or more numbers

Factors of 18: 1, 2, 3, 6, 9, 18
Factors of 72: 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36, 72

Largest number in both of the lists: 18

GCF: 18