Two charges $+6\,\mu\text{C}$ and $-3\,\mu\text{C}$ are placed at points $(-2.7\text{ cm},0)$ and $(2.7\text{ cm},0)$ respectively in an external electric field of $1.8\times10^5r^{-2}\,\text{NC}^{-1}$, where $r$ is the distance of a charge from the origin. Then the net electrostatic energy of the system of the two charges is: