Please help me to solve this.

In the diagram, http://farm3.static.flickr.com/2255/...474b2ea3_m.jpg

AD and BC are arcs of circles with centre O, such that OA = OD = r cm, AB = DC = 8 cm and angle BOC = θ radians.

Given that the area of the shaded region is 48 cm^2, show that r = 6/θ – 4.