To find the GCF of 66 and 91 you have to find the factors of each:
66 = 1, 2, 3, 6, 11, 22, 33, 66
91 = 1, 7, 13, 91
So the only, and therefore greatest, common factor is 1.
Now we do the same with 84 and 92:
84 = 1, 2, 3, 4, 6, 7, 12, 14, 21, 28, 42, 84
92 = 1, 2, 4, 23, 46, 92
This time we have multiple common factors but the greatest common factor is 4.
Now 51 and 75:
51 = 1, 3, 17, 51
75 = 1, 3, 5, 15, 25, 75
The GCF is 3
Now 64 and 72:
64 = 1, 2, 4, 8, 16, 32, 64
72 = 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36, 72
GCF is 8. You just look for the highest number that is the same.
Finally, 99 and 66:
99 = 1, 3, 9, 11, 33, 99
66 = 1, 2, 3, 6, 11, 22, 33, 66
GCF is 33.
Hope this helps.