Let's use pythagorean theorem to calculate the remaining side:
[tex]\begin{gathered} c=\sqrt[]{a^2+b^2} \\ c=\sqrt[]{7^2+3^2} \\ c=\sqrt[]{21+9} \\ c=\sqrt[]{30} \end{gathered}[/tex]The area of a square is given by:
[tex]\begin{gathered} A=s^2 \\ \text{Where:} \\ s=\text{One of its sides} \\ A=(\sqrt[]{30})^2 \\ A=30 \end{gathered}[/tex]