P.E. of two charges
$\mathrm{u}=\frac{1}{4 \pi \varepsilon_{0}} \frac{\mathrm{q}_{1} \mathrm{q}_{2}}{\mathrm{r}}$
$r=\sqrt{\left(x_{2}-x_{1}\right)^{2}+\left(y_{2}-y_{1}\right)^{2}+\left(z_{2}-z_{1}\right)^{2}}$
$=14 \mathrm{~cm}$
$\therefore \mathrm{u}=\frac{9 \times 10^{9} \times 7 \times 10^{-6} \times(-4) \times 10^{-6}}{14 \times 10^{-2}}$
$=-1.8 \mathrm{~J}$