1. The problem statement, all variables and given/known data I am very rusty on my mathematics and I am wondering if there is a way to express Rs in terms of r1 and r2. The positions of the bodies are all relative to the origin 0 (C.o.M between m1 and m2). Basically I'm trying to express the two vectors coming from m3 in terms of the masses and position vectors for m1 and m2. 2. Relevant equations R3-1 = r1-Rs R3-2 = r2-Rs 3. The attempt at a solution I think r1 and r2 are related through the expression: r2= -(m1/m2)*r1 but this was very much 'fudged' so I think I'm approaching the problem wrong. Any help much appreciated. Cheers.