$\therefore d=\frac{V}{E}$
$C=\frac{K \varepsilon_0 A}{d}$
$\therefore A=\frac{C d}{K \varepsilon_0}$
$=\frac{C V}{K E \varepsilon_0}$
$=\frac{80 \times 10^{-12} \times 12 \times 10^3}{5 \times 10^9 \times 8.85 \times 10^{-12}}$
$=21.69 \times 10^{-6}$
$=21.7 \times 10^{-6} \;m ^2$

$\left(\frac{1}{4 \pi \in_{0}}=9 \times 10^{9} Nm ^{2} / C ^{2}\right)$

If $D > > d,$ the potential energy of the system is best given $b$
