"Nem az új autó lassú, hanem a régi."

Translation: It is not the new car that is slow, but the old one.

[isa_88]

I would suggest: "It is not the new car that is slow, but the old one."

[jsiehler]

Yes, your suggestion is good. The English translation Duo gives above ("It is not the new car slow, but the old one") is not correct at all.