Asking for information or help
Notice that we often use tag question to ask for information or help, starting with a negative statement, This is quite a friendly/polite way of making a request. For example, instead of saying "Where is the police station?" not very polite, or "Do you know where the police station is?" slightly more polite, We could say: "you wouldn't know where the police station is, would you?" Here are some more examples
you don't know of any good job, do you?
you couldn't help me with my homework,could you?
you haven't got 10 to lend me,have you?