Question

if 3e charge is at the origin and the magnesium nucleus is 3cm from the origin in the +x direction, at what position along the x- axis should an electron be placed so that the electron experiences no net force?

Answer 1

What you are really asking is where to put a 1 e charge to exactly balance a 3 e located charge at the origin if we are standing at x = 3 holding charge q

F/q = k Q1 /r^2
F/q to left = k (-3)/9

F/q to right = k (-1)/(x-3)^2

so
1/(x-3)^2 = 1/3
(x-3)^2 = 3
x-3 = sqrt 3
x = 3 + sqrt 3

Answer 2

IS 1