This seems to an application of the sine rule . If you know the distance apart of the two bearing positions A and B, then the distance of a third point point X can be found from the triangle ABX using AB/sin AXB = AX/sin XBA
Thank you fresco
I was struggling with d=l sin a sinb/ sin [a+b] .where a and b are the angles at
A and B. That came from Wicki. Triangulation and seems needlessly complex even if it works .