$= U _{ f }- U _{ i }$
$=\left(- pE \cos \left(180^{\circ}\right)\right)-\left(- pE \cos \left(0^{\circ}\right)\right)$
$= pE + pE =2 pE$
$=2 \times 6 \times 10^{-6} \times 1.5 \times 10^3=18\,mJ$
$V$. Energy stored in the above combination is $\mathrm{E}$. When these capacitors are connected in series to the same supply, the stored energy is $\frac{9}{\mathrm{x}} \mathrm{E}$. The value of $x$ is___________.