(b) Two charges of $8\text{ }\mu\text{C}$ and $-4\text{ }\mu\text{C}$ are located at points $(-18\text{ cm}, 0, 0)$ and $(18\text{ cm}, 0, 0)$ respectively. (i) Calculate the electrostatic potential energy of the system of the two charges. (ii) The same system of charges is placed in a region where electrostatic potential $V(r) = \frac{\text{A}}{r}$, where $\text{A} = 9 \times 10^4\text{ V.m}$. Calculate the potential energy of the system.