1. The problem statement, all variables and given/known data The first one has charge Q is at "-a" from origin the second is at "+3a" which is an unknown charge.What are the values for unknown charge if at the origin the net electric field they produce with magnitude 2Ke(Q/a2) 2. Relevant equations Ke (Q/a2)i - Ke(q/9a2)=2Ke(Q/a2)i if charge is negative or if the charge is positive Ke (Q/a2)i - Ke(q/9a2)=-2Ke(Q/a2)i 3. The attempt at a solution The question is how to choose signs for equation. Attempt: Assuming charge one is pointed towards the origin along x axis so the sign will be "+i" and the second charge is pointed towards the origin in the negative x direction so tthe sign will be "-i". Two possible values are at the origin: "+" or "-" 2Ke(Q/a2) . If you choose "+" at the origin the answer will be -9Q. And for case chosen "-" sign in the origin, the answer will be 27Q so the particle has positive charge. I wondering if this is the explanation?