Hello karldiesenLook at the attached diagram. divides internally in the ratio if ; and divides externally in the ratio if ; or (taking account of the directions) .

Now if points and have position vectors and respectively, then the point that divides the line segment in the ratio has position vector(It's quite straightforward to prove this: begin with , and then say ... etc)

So you can simply plug the position vectors of and into this formula with and . If my arithmetic is correct, I reckon the answer is .

Grandad