This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
in my opinion, the answers to your two questions are both yes. You have to comply with the assignment instructions, and regarding the DB interface and the Data class, it states that : "Data.java must implement the DB interface".
There is no restriction on other interfaces that Data must or must not implement. And stritcly speaking, implementing a subclass of DB means implementing DB.