well, I would not say it is wrong but if you are in an exam , you should say '' whether '' because of the '' or not '' for example: I don't know if he will arrive or not ( it sounds like something despite the yes or no can happen ) but if you say > I don't know whether he will arrive or not ( he will arrive or not, there isn't another option )

I see. Thank you.

Both are correct [English Grammar in Use 5th edition, unit 50]. The "or not" is unneeded and usually omitted.