It's a BNCD

The alphabet has been shifted by one.

A→B

B→C

C→D

⋮⋮

Z→A

We can see the question is now: FIVE PLUS ONE = ?

Which we know to be six, which is translated to "RHW"

The hint gives the answer, noticing that code is written everywhere on the page, we can assume that BNCD = CODE.

Given this information, we see that NMD = ONE.

Since there is an equals sign and a number already in the code, we assume that it is an informal equation:

We assume that the operator is in the middle, making us believe that the other code piece is another number:

The only two numbers that have four letters in their names and end in E are FIVE and NINE, since we know that N = C, if the number were NINE, it would be represented as C?CD. Thus it must be FIVE. Minus, and times have five letters. Over has four but does not start with an N. Thus, if we assume it is PLUS, we should get a number with an H in the middle of it and an R at the front.

We get this, and thus the answer is RHW.

Anyway, that's the informal, intuitive guide that'll probably be removed shortly :)

TJY ( = six). I stand corrected. It's RHW.