How do you calculate dividends declaired by preparing a statement of retained earnings?

You have to know the dividends declared before you prepare the statement, not use the statement to calculate it. It depends on what information the problem has given you and what you've had to do prior to this. I don't know what is in your problem. You're going to have to provide more info than that -- I don't even know if you've been doing entries and should have been keeping track of it, or it's providing some other information somewhere. No way for us to know that.

Come to think of it, they could even provide that retained earnings statement with missing numbers, but I've never seen that in a problem before.