Ответ:

86 + 42 : (x - a) = 100; x = 11;

86 + 42 : (11 - a) = 100; 42 : (11 - a) = 100 - 86; 42 : (11 - a) = 14;

11 - a = 42 : 14; 11 - a = 3; a = 11 - 3; a = 8.