A square prism has sides of length 11cm, 11cm and 15cm as shown.

Now, we want to find $y$, the length of the diagonal $DF$.

Calculate $y$ to two decimal places.

Please directly answer the question and provide the final value, e.g., 1, 2.5, 300.
Question: A square prism has sides of length 11cm, 11cm and 15cm as shown.

Now, we want to find $y$, the length of the diagonal $DF$.

Calculate $y$ to two decimal places.

Please first conduct reasoning, and then answer the question and provide the final value, e.g., 1, 2.5, 300, at the end.
Question: A square prism has sides of length 11cm, 11cm and 15cm as shown.

Now, we want to find $y$, the length of the diagonal $DF$.

Calculate $y$ to two decimal places.
