A rectangular room has a perimeter of 42 feet and an area of 108 square feet. What is the length, in feet, of the shorter side?
Let the longer side have length $a$ and the shorter side have length $b$. We have the two equations \begin{align*}
2a+2b&=42\\
ab&=108
\end{align*} From Equation (1), we have $a+b=21$, so $a=21-b$. Substituting that into Equation (2) to eliminate $a$, we get \begin{align*}
(21-b)(b)&=108\\
21b-b^2&=108\\
b^2-21b+108&=0
\end{align*} Factoring the equation, we get $(b-9)(b-12)=0$, so $b=9$ or $b=12$. $b=12$ corresponds to the longer side, so the length of the shorter side is $\boxed{9}$ feet.