GFC stands for the Greatest Common Factor.
The factors of 9 and 81 are:
[tex]\begin{gathered} \text{Factors of 9}\colon1,3,9 \\ \text{Factors of 81}\colon1,3,9,\ldots,81 \end{gathered}[/tex]Notice that the greatest common factor they can share is 9 because that's the greatest factor 9 can have.
Thus, the answer is 9.
