(6 1/21 + y) - 12 8/21 = 3 4/21

(127/21 + y) - 260/21 = 67/21

127/21 + y - 260/21 = 67/21

- 19/3 + y - 260/21 = 67/21

- 19/3 + y = 67/21

y = 67/21 + 19/3

y = 200/21 = 9 11/21.