well lets see..
factors of 28 are 1, 2, 4, 7, 14, 28
factors of 56 are 1, 2, 4, 7, 8, 14, 28, 56
factors of 84 are 1, 2, 3, 4, 6, 7, 12, 14, 21, 28, 42,84
as u can see, 28 is the highest in all of them so the greatest common factor is 28. hope this helps!
