(3 8/21 + k) - 5 13/21 = 2 4/7
3 8/21 + k = 2 4/7 + 5 13/21
3 8/21 + k = 2 12/21 + 5 13/21
3 8/21 + k = 7 25/21
k = 7 25/21 - 3 8/21
k = 4 17/21
------------------------------------------
Проверка: (3 8/21 + 4 17/21) - 5 13/21 = 2 4/7
7 25/21 - 5 13/21 = 2 12/21 = 2 4/7