The remainder should be an integer and you have that one line from the bottom.
a + b = 28x - 14y - 2 = 14k - 2, (where k = 28x - 14y).
The remainder after division by 14 is -2.
If you don't like the negative sign you can simply add 14 to get a remainder of 12.
Check it out with some random substitutions for a and b if you wish.
For example, let a = 74 (= 5*14 + 4),
let a - b = 38 (= 2*14 +10),
then b = 74 - 38 = 36,
so a + b = 74 + 36 = 110 = 98 + 12 = 7*14 + 12, remainder 12,
(or 110 = 112 - 2 = 8*14 - 2, remainder -2).