Two point charges of +2.0 μC and -6.0 μC are located on the x-axis at x = -1.0 cm and x 12) = +2.0 cm respectively. Where should a third charge of +3.0-μC be placed on the +x-axis so that the potential at the origin is equal to zero?

 x = 0.006 m

Explanation:

The potential at one point is given by

          V = k ∑

remember that the potential is to scale, let's apply to our case

          V = k (q₁ / x₁ + q₂ / x₂ + q₃ / x)

in this case they indicate that the potential is zero

          0 = k (2 10⁻⁶ / (- 1 10⁻²) + (-6 10⁻⁶) / 2 10⁻² + 3 10⁻⁶ / x)

         3 / x = + 2 / 10⁻² + 3 / 10⁻²

         3 / x = 500

          x = 3/500

          x = 0.006 m

there are no contact forces mentioned on the list of choices that you've provided.