Volume = area of cross section x length
A cross section is the shape we get when cutting straight through an object. It is like a view into the inside of something made by cutting through it. e.g. the cross section of a triangular prism is a triangle.
https://www.youtube.com/watch?v=cC0fZ_lkFpQ
Watch this video to revise what a circle is and all of it's properties. This will introduce you to the concept of PI and will help you with question 3.