(5/7-2/3у)×21=1
(5/7-2/3у)=1÷21
5/7-2/3у=1/21
-2/3у=1/21-5/7 | ×3
-2/3у=1/21-15/21
-2/3у= - 14/21
у= - 14/21:(- 2/3)= 14/21: 2/3=14/21×3/2= 14×3/21×2 | 14:2/21:3= 7×1/7×1
у= 7/7=1
Ответ: у=1
Проверка.
(5/7-2/3×1)×21=1
(5/7-2/3)×21=1
(5/7 | ×3 - 2/3 | ×7)×21=1
(15/21-14/21)×21=1
1/21×21=1
1×21/21=1
21/21=1
1=1
Вроде так