Bonjour

a. 21/6 = (3 x 7) / (3 x 2) = 7/2

b. - 14/12 = (2 x - 7) / (2 x 6) = - 7/6

c. - 48/18 = (6 x - 8) / (6 x 3) = - 8/3

d. 15/45 = (15 x 1) / (15 x 3) = 1/3.