Answer
$9$
Work Step by Step
Factor each number completely to obtain:
$63 = 7 \cdot 3 \cdot 3
\\81 = 3 \cdot 3 \cdot 3 \cdot 3$
Since two 3s are common to both, the greatest common factor of the two given numbers is:
$3 \cdot 3 = 9$
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