Video: David Cameron vows to 'do extra homework' after Letterman's history quiz David Cameron said he needed to do 'extra homework' after he was unable to named the composer of Rule Britannia! or give the correct translation for 'Magna Carta' on the David Letterman chat show. The Prime Minister was speaking in Brazil, during a visit to open a JCB construction vehicle assembly plant near Sao Paulo. Asked how he rated his performance on the Letterman show, Mr Cameron said: "I'm a history obsessive, so I'm sorry I didn't do better. I think, when I get home and do my children's homework, maybe I need to sit down and do a little bit extra myself.