What is the coordinate system of Bing Maps

Dear All,

I am using Bing maps as a base map in flex viewer.my operation layer have the coordinate system WGS 84.it is covering the extent of saudi arabia.But when i put this in flex viewer and run the application it zoom to the some where in algeria.As per my understanding the coordinate system difference.Please let me know who to zoom to the my required extent.

thanks in advance.