a. 9/7 + 6/14 = 9 x 2 / 7 x 2 + 6/14 = 18/14 + 6/14 = 24/14 = 12/7

b. 9/7 - 18/21 = 9 x 3 / 7 x 3 - 18/21 = 27/21 - 18/21 = 9/21 = 3/7

c. 2/5 + 7/20 = 2 x 4 / 5 x 4 + 7/20 = 8/20 + 7/20 = 15/20 = 3/4

d. 2/5 - 20/25 = 2 x 5 / 5 x 5 - 20/25 = 10/25 - 20/25 = -10/25 = -2/5