Table is the subject of the sentence, so it's nominative! In Latin, like many other languages that utilize cases (nominative, accusative, genitive, etc.), there are possessive pronouns that effectively take the role of genitive. YOUR table, where your is the possessive pronoun!
If it were the brother's table, it would be "mensa fratris" where "fratris" is brother in the genitive case (nom. is "frater"). Hope this helps!