What is the greatest common denominator of 81, 108, and 117?

If you mean greatest common factor then that means

factor each and group the ones that are same

81=3 times 3 times 3 times 3

108=2 times 2 times 3 times 3 times 3

117=3 times 3 times 13

so we look for the group common to all (I bolded it)

81=3 times 3 times 3 times 3

108=2 times 2 times 3 times 3 times 3

117=3 times 3 times 13

the answeis 3 times 3=9

greates common denomenator of these nubers is 9