You have two identical point charges of +6.0 nC each, one at the position, x=+3.0cm, y=0, z=0, and the other at, x=-3.0cm, y=0, z=0. Find the electric potential due to these charges at x=+8.0, y=0, z=0.

A point charge is at the fixed position x=0, y=0, z=0. You find that you must do 8.10*10^-6J of work to bring a second point charge from infinity to the position x=+4.0cm, y=0, z=0. What is the value of the second charge?