The gcf for the numbers 28, 56 and 84 is 28
Since
28: 1, 2, 4, 7, 14, 28
56: 1, 2, 4, 7, 8, 14, 28, 56
84: 1,2,3,4,6,7,12,14,21,28,42,84
Since the biggest number they all have is 14 so the gcf is 14.
The gcf for the numbers 30, 45, and 75 is 15
30: 1,2,3,5,6,10,15,30
45: 1,3,5,9,15,45
75: 1,3,5,15,25,75
Since the biggest number they all have is 15 so the gcf is 15.
Hope this helped