Question on Pre-Calculus/Geometry Problem?

This is how the problem reads:Geometry - A rectangle is bounded by the x-axis and the semicircle y = square root of "36 - x^2." Write the area of the rectangle as a function of x, and determine the domain of the function.You can find a scan of the problem here:http://i8.tinypic.com/4katavs.jpgI'd love it if somebody can explain to me how to solve it and not just the answer. Thank you guys so much!