Question

Part A & B are 0.10m apart. A point charge of +3.0*10^-9C is placed at A and a point charge of +1.0*10^-9C is placed at B. X is a point on the straight line through A and B. If a third charge C or charge 1.5*10^-9 is placed at X, where must X be for the resultant force to be zero.