How to find the greatest common factor of 14 42 91?

Each number is first written as the sum of its prime factors. Here we have ${\displaystyle 14}$, ${\displaystyle 42}$ and ${\displaystyle 91}$ and their prime factors are
${\displaystyle 14=2\times 7}$
${\displaystyle 42=2\times 3\times 7}$
${\displaystyle 91=7\times 13}$
Now list all the common factors (if any prime is occurring more than one time in each number pick up as many times it is spearing in all the numbers).
In above case, however, we have just ${\displaystyle 7}$, which repeats in all.
Hence, here ${\displaystyle 7}$ is greatest common factor.