Let's draw the figure:
Cutting the other side, it appears to be a right triangle. Thus, to be able to get the length of its diagonal, we can use the Pythagorean Theorem.

![\text{ c = }\sqrt[]{a^2+b^2}](https://img.qammunity.org/2023/formulas/mathematics/college/snwjlnky39t9kxlnrd249m0l3gizvkff3i.png)
Let's substitute a and b by 15 since the figure is a square, thus, all of its sides are the same.
We get,
![\text{ c = }\sqrt[]{a^2+b^2}](https://img.qammunity.org/2023/formulas/mathematics/college/snwjlnky39t9kxlnrd249m0l3gizvkff3i.png)
![\text{ c = }\sqrt[]{15^2+15^2}](https://img.qammunity.org/2023/formulas/mathematics/college/zcluboagmjqp2xltbqvjeyzmwshz6rxayk.png)
![\text{ c = }\sqrt[]{225\text{ + 225}}](https://img.qammunity.org/2023/formulas/mathematics/college/i5jlthzggslzcphzdvnq0r848kqm2zgdt1.png)
![c\text{ = }\sqrt[]{2\text{ x 225}}\text{ = }\sqrt[]{2x15^2}](https://img.qammunity.org/2023/formulas/mathematics/college/8xmaevlbjv05h5cujrf5vd0rlvql2cu18h.png)
![\text{ c = 15}\sqrt[]{2}](https://img.qammunity.org/2023/formulas/mathematics/college/xwb5zhza3gkh5nhwjttf88a40f95ajzu52.png)
Therefore, the length of its diagonal is 15√2 inches.