Where are the measurements m1 and m2 in your formula?

The distance between the device and the center of the two circles can be also calculated as: (m2r1 - m1r2)/(m2-m1)

Sorry, I forgot to specify what alpha is. The angle alpha is the angle between the red line and a line that connects the device with the common center of the two circles.

If the problem is a 2D one, then the angle can be computed using this equation: cos(alpha) = (r1-r2)/(m2-m1)