Two charged particles are fixed to an x axis: particle 1 of charge q1 = 2.1 x 10^-8 C 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 at infinity) is the net electric field produced by the two particles equal to zero? (please show all work and explain. Thank you!!)