Translation:Surely Marcus does not live in New York?
Novi Eboraci here is the locative case meaning "in New York". In general you only use it with cities, towns, small islands, and only a couple of other nouns (domus being one) and usually denotes the location something 'occurs'. Other nouns you just use in followed by the ablative form.
Novum Eboracum is the accusative case. It is usually used as the direct object of a verb (the thing being acted up).
I answered "Marcus doesn't live in New York?" which was marked incorrect. Is it safe to assume "num" in a sentence requires the use of "surely" or should I report my translation as correct?
I looked up the meaning of "num":
adverb if, whether; now, surely not, really, then (asking question expecting neg)