Answer
$12$.
Work Step by Step
The given numbers are
$72,84$
Prime factorization of each number is
$72=2\cdot 2 \cdot2\cdot 3 \cdot 3 $
$84=2\cdot 2 \cdot3\cdot 7$
Common prime factors are
$=2\cdot 2 \cdot3 $
Simplify.
$=12$
Hence, the greatest common factor is $12$.
