J'arrive pas l'EX 1. HELP !

bonjour

a = 1/3 - 4/21
a = 7/21 - 4/21 =  3/21 = 1/7

b = 6/5 : ( 1/15 - 1/5)
b = 6/5 : ( 1/15 - 3/15)
b = 6/5 : - 2/15
b = 6/5 * - 15/2
b = - 90/10 = - 9