1. The problem statement, all variables and given/known data 2. Relevant equations See #1 3. The attempt at a solution Based on the information in the question, I did what I could, but in the back of the book they have an answer of -9.9a and I have no idea how? For the +3q on the right, I set the denominator to (2a+r)^2, since its distance would be the square of both the 2a between them as well as the additional r distance between the point and -2q. I am thinking they somehow managed to relate a to r to get rid of r, but I can't seem to figure out how I can do that or in some other way get rid of r?