Homework 22.11

Two charged particles are fixed to an x axis: Particle 1 of charge q1 = 2.1 x 10 - 8C is at position x = 20 cm and particle 2 of charge q2 = - 4.00q1 is at position x = 70 cm. At what coordinate on the axis (other than infinity) is the net electric field produced by the two particles equal to zero?