(x+4/21)-4/15=16/35 это дроби
(х + 4/21) - 4/15 = 16/35
х + 4/21 = 16/35 + 4/15 = 48/105 + 28/105 = 76/105
х = 76/105 - 4/21 = 76/105 - 20/105 = 56/105
х = 56/105
Проверка: (56/105 + 20/105) - 28/105 = 76/105 - 28/105 = 48/105 = 16/35
Ответ: х = 56/105