> Write down a statement which will guarantee that one of these robots who can make such a statement must be the rich good robot.
That's a little unclear, but I think they mean that it should be a statement that the rich robot ... and only the rich robot ... can make.
HINT
For the first part, you just need a natural language statement like "I am a rich robot"
Of course, that statement does not work, since both the rich good robot, as well as the bad robot could be making that statement.
So, try and find a statement that can only be made by the rich robot.