Translation:Let's go to the museum.
Apparently, you read "박물관" as "방물관". (as says Naver)
Yes, it's a rule: ㄱ+ㅁ --> ㅇ+ㅁ between syllables/letter blocks.
OK, so we add ㅂ시 to the verb to say "let's", is that right?
You can also use 가예요 to make it conversational.
But the chapter is about fornal usage
Do you mean 가요?