Quaternion offset new location

Hi,

So I have a person (A) who's rotation is given with a quaternion (his location is given too). Now I want to figure out the location of a second person (B) that's standing in a certain position in relation to A. Like this:

(^ means a person that's looking in that direction)

---^---

-----^-

So the second person is standing behind the other person at a certain angle (alpha) at a certain distance (x). What I want to find is the location of person B. I can't for the life of me figure out how to calculate this with quaternions, even though I realise it's kind of simple o.O.

Thanks!