Question:
A rectangular patio has an area of $180$ square feet and a perimeter of $54$ feet. What is the length of the diagonal (in feet) squared?

Answer:
Set one side of the patio equal to $a$ and the other equal to $b$, producing two equations: \begin{align*}
ab&=180,\text{ and}\\
2a+2b&=54.
\end{align*}The second equation can be rewritten as $b=27-a$. Substituting, we have \begin{align*}
180&=a\left(27-a\right) \quad \Rightarrow \\
180&=27a-a^2 \quad \Rightarrow \\
-180&=a^2-27a \quad \Rightarrow \\
0&=a^2-27a+180 \quad \Rightarrow \\
0&=\left(a-12\right)\left(a-15\right).
\end{align*}So $12$ feet and $15$ feet are the lengths of the two sides of the patio. Therefore, the diagonal is $\sqrt{12^2+15^2}$, or $\sqrt{369}$. Therefore, the length of the diagonal squared is $\boxed{369}$.