using geometric formula evaluate 4-x and 2x-8 when integral is from x=0 to 6

Answer 1

not sure what you mean by

evaluate 4-x and 2x-8

Maybe a look at the graphs will help clarify your thinking:

http://www.wolframalpha.com/input/?i=plot+y%3D4-x%2C+y%3D2x-8