About Calculating Angle

Question

Hi,

 

I have an arc where I know start point, end point and center of arc. Now I want to calculate angle from start point to end point from center point. Its like calculating angle between two points of circle. 

 

I searched for many links. I also tried math Math.Atan2 function but It not gives me expexted result that I want. Angle between 0 to 360. I want to calculate this value.

 

So please help me in this issue. Thanks for help in advanced.