Area of Semi-Circle region from Chord

http://img261.imageshack.us/img261/7381/problemgq0.png

Hello,

I would like to know how I am supposed to approach this problem.

A small semi-circle is inserted into a bigger one.

The only measurement that I have been given is that chord WX = 36mm, and is parallel to diameter YZ.

I need to find the area of the grey area, and I am completely stuck for any idea on how to tackle this with only the one measurement.

p.s. The diameter of the smaller semi-circle is not necessarily half the diameter of the larger one.

Thanks if anyone can help.