Learn Arabic (Modern Standard): لا، هي مدينة جديدة.

In Arabic, all nouns are either masculine or feminine; there is no neutral 'it'. Because the word for city, مدينة, is feminine (indicated by the Taa Marbuuta at the end), we must use the feminine pronoun هي to refer to it.

Where is the word for 'is' in this sentence?

Arabic does not use a present tense verb for 'to be' (is/am/are). Instead, you simply put the subject (هي) and the information about it (مدينة جديدة) next to each other. This creates a nominal sentence.

How do we know it means 'a new city' and not 'the new city'?

Neither مدينة (city) nor جديدة (new) has the definite article ال (al-) attached to the beginning. When a noun is written in its bare form like this, it is indefinite, which translates to 'a' or 'an' in English. If you read it aloud with full endings, you would hear a 'tun' sound at the end of both words (Tanween), which is the grammatical mark of this indefinite state.

Why does the adjective جديدة (new) come after the noun it describes?

In English, we say 'a new city', putting the adjective first. In Arabic, adjectives always follow the noun they describe. Also, notice that because مدينة is an indefinite noun, the adjective جديدة must also be indefinite (meaning it also does not have ال at the beginning) so that they match perfectly.