"Num urbs Novum Eboracum in California est?"
Translation:Surely the city of New York is not in California?
Novum Eboracum is in the nominative because it is an appositive elaborating on the word urbs. The English use of "of" isn't possessive (genitive in Latin) but rather a shortening of "by the name of."
I'm not a native speaker but saying "in the city New York" (city [called] New York) maybe sounds like a bad English. Maybe "New York city" could be accepted. In Latin you would say it like this - in nominatives.
The "correct" answer given - "Surely the city of New York is not in California?" would require 'New York' to be in genitive case, i.e. 'Novi Eboraci'. As it is, I translated "urbs Novum Eboracum" as being nouns in apposition, i.e 'the city, New York, ... ' .
...the city, New York,... should be accepted, so long as everything around it is correct. If you report it the course creators can fix it.
Fair enough. How do you 'report' a possible new answer long after you have completed the lesson? (I'm new to Duo:)