The distance between opposite corners refers to the diagonal, which we can find using Pythagorean's Theorem where the diagonal is the hypotenuse and the given dimensions are the legs.
![\begin{gathered} c^2=a^2+b^2 \\ c^2=(9m)^2+(12m)^2 \\ c=\sqrt[]{81m^2+144m^2} \\ c=\sqrt[]{225} \\ c=15 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/dsnn0d236wvbed1q7eo2aaaqpsyagn63jj.png)
Hence, the distance between opposite corners of the living room is 15 meters.