What is the electric potential 3.0 cm away from a $-12\text{nC}$ point charge?

A $-9.0\text{nC}$ charge sits at the origin, and a $+18\text{nC}$ charge is at $x=2.0\mathrm{cm}$. At what location(s) is the potential zero?

A uniform electric field with strength $12\mathrm{V}/\mathrm{m}$ points from point A to point B, which are on the x-axis: $x_{\text{A}}=45\mathrm{m}$ and $x_{\mathrm{B}}=12\mathrm{m}$. What is the potential difference $V_{\text{A}}-V_{\mathrm{B}}$ ?

A spherical balloon is charged so that the potential on its surface is $40\mathrm{V}$. The balloon is now deflated to half its original radius (with no loss of charge). What is the new potential on its surface?

Metal sphere A is charged until it has a potential of $200\mathrm{V}$. Metal sphere B is initially uncharged, and has a radius twice the radius of sphere A. A small wire is used to connect spheres A and B, so that they form one continuous equipotential surface. What is the final potential on the surface of sphere B?