Two charges of magnitude $5 nC$ and $-2 nC$ are placed at points $\left(2 cm, 0, 0\right)$ and $\left(x cm, 0, 0\right)$ in a region of space, where there is no other external field. If the electrostatic potential energy of the system is $-0.5 \mu J$. The value of $x$ is (in m)