pls help me with the attached question
Krizalid started you off. For the final question, I don't really want to go through it in detail because that would be like reinventing the wheel. we have done volumes of revolutions scores of times on this site, just do a search for "shell method" or "disk method"
several users, including myself, have gone into great detail with volumes of revolutions and, in addition, have posted links to websites for more information. check out the threads.
(of course, if there is something you don't understand, you can still say so, but please try to understand before asking questions)