How should I begin this question?

[omega16]

If alpha is a quadratic integers in Q[sqrt(-d)] , then define a notion of congruence (mod alpha).
Furthermore, define +, -, and X for congruence classes , and show that this notion is well-defined.

Could you please guide me to begin this question? Thank you very much.

 

[HallsofIvy]

Do you know how to do that for ordinary integers? If so, just follow the same basic idea.