You have to figure out the highest common factor of each set of values:
9a + 21 = 3(3a + 7)
21b - 49 = 7(3b - 7)
54 - 6c = 6(9 - c)
8a + 32b = 8(a + 4b)
4p + 28q + 8r = 4(p + 7q +2r)
84a - 36b -12c = 6(14a - 6b - 2c)
6p + 9q + 15r = 3(2p + 3q + 5r)
18s - 30t + 54u = 6(3s - 5t + 9u)
Hope this helps :)