I know I can express the distance from a point on the z-axis from the charge 2q as sqr(2a2+z2). But I don't know how to express the distances to the other charges. I guess I could start by saying: Let A be the point where charge q sits, B the point where charge 2q sits. Then the distance from a point on the z-axis to charge q would be sqrt((|AB|-a) 2+a2+z2) and then do the same for the other charges. But this seems like a lot of work. I wouldn't imagine I'm supposed to start making up names for the points. Am I barking up the right tree with this method?