If you start from then I agree with your teacher that However, if you start from then you can't say that unless, in addition, you know that is always positive.
The rules come down to context. From where do you start? Is there some physical principle involved that allows you to rule out one sign possibility? In general, if you have the equation then without any other context, you must always allow the solutions However, if you know ahead of time, say, that and then you can rule out the negative solution and say that
Hope that helps.