3/9 + z = 4 6/12. y - 2 5/7 = 3 1/9

3/9 + z = 4 6/12
z = 4 6/12 - 3/9
z = 4 1/6
проверка: 3/9 + 4 1/6 = 4 6/12
                  4 6/12 = 4 6/12

Y - 2 5/7 = 3 1/9
y = 3 1/9 + 2 5/7
y = 5 52/63
проверка: 5 52/63 - 2 5/7 = 3 1/9
                     3 1/9 = 3 1/9