(287 + х ) - 109 = 298287 + х = 298 + 109287 + х = 407х = 407 - 287х = 120Проверка :(287 + 120) - 109 = 298 407 - 109 = 298 298 = 298
(287+х)-109=298287+х=298+109287+х=407х=407-287х=120.