FAIRE QUE LE A ET B MERCI A TOUS

bonsoir

a = 6/7 - 20/14
a = 12/14 - 20/14 = - 8/14 = - 4/7

b = ( 3/4 - 4) : ( 3/4 + 1/3) 
b = ( 3/4 - 16/4) : ( 9/12 + 4/12)
b = - 13/4 x 12/13
b = - 156/52= - 3