Пошаговое объяснение:

3/7 + 1/6 + 7/6 + 6/7 = 3/7+6/7+1/6+7/6 = 9/7 + 8/6 = 1 + 2/7 + 1 + 1/3 = 2 + 6/21 + 7/21 = 2 13/21

(3/7+6/7)+(1/6+7/6)=9/7+8/6=54/42+56/42=110/42=2 13/21