Bonjour !
a) 3/11 - 4/12
= 3x12/11x12 - 4x11/11x12
= 36/132 - 44/132
= -8/132
= -2/33
b) 3/11 + 4/12
= 3x12/11x12 + 4x11/11x12
= 36/132 + 44/132
= 80/132
= 20/33
c) 5/7 - 6/18
= 5x18/7x18 - 6x7/7x18
= 90/126 - 42/126
= 48/126
= 16/42
= 8/21
Voilà !
Bonne journée !