In order to calculate the value of 'a', we can use the Pythagorean theorem, where the hypotenuse squared is equal the sum of each leg squared:

So if the hypotenuse is 12 and the legs are 9 and 'a', we have:
![\begin{gathered} 12^2=9^2+a^2 \\ 144=81+a^2 \\ a^2=144-81 \\ a^2=63 \\ a=\sqrt[]{63} \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/qjiq1yunk9kmuqzyaohl9xmtexeslfghgx.png)
So the number beneath the radical symbol is 63.