Answer :

AL2006

-- The factors of  5  are  1  and  5 .

-- The factors of  12  are  1,  2,  3,  4,  6, and  12.

-- The factors of  14  are  1,  2,  7, and  14.

The only factor that's common to  5,  12, and  14 is  ' 1 ',
so naturally it must also be the greatest common one.