$C=\frac{A \varepsilon_{0} \varepsilon_{r}}{d}$ $......(I)$

Substitute $15 \times 10^{-9}$ for $C, 8.85 \times 10^{-12}$ for $\varepsilon_{0}$ and $2.5$ for $\varepsilon,$ in equation $(I).$

$15 \times 10^{-9}=\frac{A\left(8.85 \times 10^{-12}\right)(2.5)}{d} \quad \ldots \ldots$ $(II)$

The expression for electric field in given by,

$E=\frac{V}{d}$

$30 \times 10^{6}=\frac{30}{d}$

$d=10^{-6} m$

Substitute $10^{-6} m$ for $d$ in equation $( II )$.

$15 \times 10^{-9}=\frac{A\left(8.85 \times 10^{-12}\right)(2.5)}{10^{-6}}$

$A=\frac{15 \times 10^{-9} \times 10^{-6}}{8.85 \times 10^{-12} \times 2.5}$

$=6.7 \times 10^{-4} m ^{2}$