"Those boys are not new."
Translation:Ti kluci nejsou noví.
Probably you're not that far in the course yet, but maybe the Tips & Notes from the later skill ''Demo'' (Demonstratives) will already help you: https://www.duolingo.com/skill/cs/Demonstratives/tips-and-notes
You must always report the complete sentence. There is no point arguing about a single word. See https://forum.duolingo.com/comment/27749621/So-many-different-ways-of-saying-things-hoch-kluk-chlapec