"Are the cars yours?"
Translation:Arabalar senin mi?
The question word can only get personal endings and the past tense markers.
"senin" is in the genitive case and rightfully belongs on "senin" which can be "your" or in this case "yours." If you say "misin" on the end, it would be asking something like "Are you the cars?"
I've just made the same mistake. But when you think about it, it's pretty obvious, what we did wrong. Cars are the subject of the sentence, you add the suffix "-ı" in accusative case, like in a sentence "Arabaları kim kullanıyor?", where cars are the object of the sentence, hence accusative case.