\(\therefore d=\frac{V}{E}\)

\(C=\frac{K \varepsilon_0 A}{d}\)

\(\therefore A=\frac{C d}{K \varepsilon_0}\)

\(=\frac{C V}{K E \varepsilon_0}\)

\(=\frac{80 \times 10^{-12} \times 12 \times 10^3}{5 \times 10^9 \times 8.85 \times 10^{-12}}\)

\(=21.69 \times 10^{-6}\)

\(=21.7 \times 10^{-6} \;m ^2\)