Particle A carrying a charge of \(3.0 \mathrm{nC}\) is at the origin of a Cartesian coordinate system.

(a) What is the electrostatic potential (relative to zero potential at infinity) at a position \(r=4.0 \mathrm{~m}\) from the origin? 

\((b)\) With particle A held in place at the origin, how much work must be done by an outside agent to bring particle \(\mathrm{B}\), also carrying a 3. 0-nC charge, from infinity to \(r=4.0 \mathrm{~m}\) ?

(c) Particle B is returned to infinity, and particle A is moved to \(r=4.0 \mathrm{~m}\) and held there. How much work must be done by an outside agent to bring particle B from infinity to the origin?