Having a bit of trouble with what should be a fairly simple problem. Unfortunately it's been a while since I've done any of this stuff so I was wondering if any of you could help me out.

You really need to see the diagram to understand the question so I've uploaded the problem here: tinypic.com/r/5ckej5/5 (I've also attatched it)

Now in part a) it's obvious that to get the answer your simply multiplying y=9-(1/4)x^2 by 2x. What I don't understand is how that would give you the area of the cuboid.

Any help would be much appreciated thanks .