Hello, Rimas!
Another approach . . .
If and are integers such that ,
what are the possible values of ?
[a] -3 only . . [b] 0 only . . [c] only multiples of 3
. . . [d] any integer . . [e] none of these
We are given: .
Then
Therefore, must be a multiple of 3 . . . answer choice (c)