There are a number of ways to say this, but I don't think the exercise is asking you to take a stab at vocabulary you don't yet know. I think they are probably just expecting you to answer something like 辞書は301教室にあります。Sorry if that's a spoiler. If you haven't yet learned 301号 or 301号室, or 301番, etc..., then I'm sure the above answer is fine.