To obtain the greatest common divisor of (g.c.d) we must do it in simultaneous decomposition.
This method consists of extracting the common and uncommon prime factors, then
![\large\displaystyle\text{$\begin{gathered}\sf \left.\begin{matrix} \blue{12 \ \ \ 2 \ \ \ 84}\\ \ 6 \ \ \ 1 \ \ \ 42 \end{matrix}\right|\begin{matrix} 2\\ \: \end{matrix} \end{gathered}$}](https://img.qammunity.org/2023/formulas/mathematics/high-school/5axjpxmrlau3g8ppnvlgj7kbl54912n881.png)
As we can see the number 2, is the only common prime number.
The greatest common factor of 12, 2, and 84 is 2.