i think that should be accepted as well.
For the record, not possible is אי אפשר pronounced "ee efshar"
בלתי אפשרי more literally means "non possible," ie "impossible" and is an adjective, whereas אפשר is an adverb
however, in English we could say "not possible" or "impossible" and it would be the same. I am not sure if these are interchangeable or not in Hebrew. I suppose technically אי אפשר would reference a verb, and בלתי אפשרי would reference a concept/noun.