Svp aider moi c est un devoir pour demain

bonsoir

a = ( 5/6 - 2/6) - ( 7/12 + 20/12)
a = 2/6 - 27/12
a = 4/12 - 27/12
a = - 23/12

b = ( - 24/42 + 35/42) + ( 3/42 + 8/42)
b = 11/42 + 11/42 = 22/42 =  = 11/21