(3/7 - 1/21) * 14 - 1/6 СРОЧНО
3/7-1/21=общ.множ21=9-1/21=8/21 8/21*14=112/21 112/21-1/6=общ.множ.42=224-7/42=217/42=сокращаем на 7=31/6=5 1/6