9 and any two multiples of it. There are an infinite number of correct answers.
For example: 9, 18, and 27.
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The GCF of 9 and 21 is 3.
2 numbers have a gcf of 3 = 3 & 6, 6 &9,...
9, 12, and 15 have the gcf of 3. 9: 1,3,9 12: 1,2,3,4,6,12 15: 1,3,5,15
By treating it as a positive number. Since negative numbers include the factors of their positive counterparts and all positive numbers are greater than all negative numbers, the GCF will always be positive. The GCFs of (3, 9), (3, -9), (-3, 9) and (-3, -9) are all the same: positive 3.
The GCF is 9.