I don't really know what to say to introduce myself other than saying that i am a 19 year old, living in brazil who needs very much help!! Sorry, my english is not so good but i try for you to understand.
The book says that i need to proof that this formula is true with all values for x:
(5 cos x-sin x)2 = A cos 2x + B sin 2x+ C
The book also says that A,B,C is a whole number and not decimal or fráction.