Translation:Your grandparents live in a valley, don't they?
I ask my questions like "X, yes?" instead of "X, no?". Duolingo wants the latter format. Who is correct?
Negative and positive question tags work pretty much the same in Esperanto as they do in English. Generally, you'd use ĉu ne? when you're already fairly sure of something and you're just seeking confirmation, but you use ĉu? when you're checking something that you're really not sure of, perhaps something you've just heard of for the first time.
That's a general guideline governing millions of speakers in billions of situations, and different people may use the language differently, perhaps because of regional variations, some specific context, or just personal style.
For the purpose of completing the course, it's best to stick with translating positive —> positive and negative —> negative for these question tags. When you've spent a few hundred hours chatting in Esperanto and getting a feel for how the language is used in practice, feel free to Do It Your Way™. :)