The greatest commonest factor of these numbers can be found by firts writing out their factors.
84 = 1 x84, 2 x 42, 3 x 28, 4 x 21, 6 x 14, 7 x 12,
21 = 1 x 21, 3 x 7
So all the factors are
84 = 1, 2, 3, 4, 6, 7 , 12, 14, 21, 28, 42, 84
21= 1, 3, 7, 21
There it is clear that the greatest common factor is 21
