Almost correct. Собака - она, жен. род -> ела did what? - accusative -> еду whose food? - genetive -> кошки "ел кошку еды" means "ate cat of food".
У (сущ., род. п.) есть... means At (noun) there is/are... So У номера (номер is a room in a hotel) есть красный диван(диванов is genetive plural of диван and it is not correct here) means "At the room there is a red couch". You should say В номере есть красный диван - (lit. in the room there is a red couch).