Exercício: Utilize A ou AN corretamente

Good afternoon folks!

I was studying some grammar with a book called "Graded Exercises in English" by Robert J. Dixson which is a great book by the way. In one exercise about an x an,there was the sentence "It is a long trip". I did the exercises then I looked up for the answer keys and according to the book both "a" and "an" are correct in this case. Also, according to the book, "It is a one-story building" / "His cars is an used car". Could you guys explain to me why this is grammatically correct?

Guys and also why a European and an FBI agent is correct please.