6/7-(2/3-1/7)=6/7+1/7-2/3=1-2/3=1/3
6/7 - ( 2/3 - 1/7 )= 6/7 - 2/3 + 1/7= 6/7 + 1/7 - 2/3= 1 - 2/3= 1/3
=6/7+1/7-2/3=1/3