Fraction Irreductible svp

bonjour

a = 6/7 - 8/9 : 4/3
a = 6/7 - 8/9 * 3/4
a = 6/7 - 24/36
a = 216/252 - 168/252
a = 48/252 = 4/21

b = 1 - ( 1/4 + 12/20)
b = 1 - ( 5/20 + 12/20)
b = 1 - 17/20
b = 20/20 - 17/20 = 3/20