The work done for rotating a dipole is given by the change in the potential energy. The potential energy is given by

$U=-\overrightarrow{p}·\overrightarrow{E}$

$\mathrm{W} =∆\mathrm{U} = {\mathrm{U}}_{\mathrm{f}}-{\mathrm{U}}_{\mathrm{i}}$

So, the work done is 

 $$\begin{gathered} W=-pE\cos 180°-(-pE\cos 0°)=2pE \\ =2\times 6\times {10}^{-6}\times 1.5\times {10}^3=18 \mathrm{mJ} \end{gathered}$$