I have such a problem:

a) Find

(Make sure your answer is an algebraic expression with square roots but without trigonometric or inverse trigonometric functions.)

Well, I have got the answer (how could I not, with a calculator it wasn't a big deal)

but I'm not sure about this question - I don't really understand what I need to do?

b) Express in terms of

do I have to plug in the value obtained in a) or what

, doesn't make sense for me...