Реши уравнение и сделай проверку
6 1/7 - (c+ 2 4/7)=2 5/7

6 1/7 - (с + 2 4/7) = 2 5/7
с + 2 4/7 = 6 1/7 - 2 5/7
с + 2 4/7 = 43/7 - 19/7
с + 2 4/7 = 24/7
с + 18/7 = 24/7
с = 24/7 - 18/7
с = 6/7

Проверка:
6 1/7 - (6/7 + 2 4/7) = 2 5/7
6 1/7 - 3 3/7 = 2 5/7
2 5/7 = 2 5/7