If y is an even number, what are the two nearest even numbers? The two nearest odd numbers?

I suggest you try substituting an even number for y, and see what the nearest even numbers are. For example, if y = 6, what are the two closest even numbers to 6? What if y was 10, or 104? Can you figure out an expression for the two nearest even numbers in terms of the value of y that works for all of these examples?

(a-5)-(2a-3)