Х/72-13/72=29/72 и (29/42-а) - 13/42=11/42

x / 72 - 13 / 72 = 29 / 72;

x - 13 = 29;

x = 29 + 13;

x = 42;

Ответ: 42;

(29 / 42 - а) - 13 / 42 = 11 / 42;

29 / 42 - a - 13 / 42 = 11 / 42;

29 / 42 - 13 / 42 = 8 / 42;

8 / 42 - a = 11 / 42;

-a = 11 / 42 - 8 / 42;

-a = - 5 / 42;

a = 5 / 42;

Ответ: 5 / 42;