One way to find the GCF of 123 and 66 is to compare the prime factorization of each number. To find the prime factorization, you can follow the instructions for each number here:
Here is the prime factorization of 123:
And this is the prime factorization of 66:
When you compare the prime factorization of these two numbers, you can see that there are matching prime factors. You can now find the Greatest Common Factor of 123 and 66 by multiplying all the matching prime factors to get a GCF of 123 and 66 as 9:
Thus, the GCF of 123 and 66 is: 9