Question

Three point charges are arranged along the x axis. Charge q1 = -4.90nC is located at x= 0.200m and charge q2 = 2.80nC is at x= -0.350m. Where along the x-axis can a third point charge q3 = 3.44nC be placed so that the net force on it is 0N.

Answer 1

so you know that the force on q3 by q1 has to equal the force on q3 by q2. and you know that x1+x2=0.550m. You have two equations and two unknowns, so solve for one of the distances and add or subtract that from the x coordinate relevant to that distance to find the x coordinate of q3