Q:

how to get point on axis of a line in distance d

I have two points A,B so they form a line AB. I'd like to know how to calculate a point in distance d on axis of this line. 
This question gives the answer for rotating a point in 2D: c# - how to move point a given distance d (and get a new coordinates)
So I can get the mid of line AB and rotate it. But I need to find out, by how many degrees I need to rotate - 90 or 180. 
How can I do it in 3D/gps coords?
http://postimage.org/image/bt9zdway3/
any ideas?

A:

I've calculated center C of line AB then then translated C to desired distance C' and rotated C' around original C. Probably there is easier way but it worked for me

