\begin{exerciseStatement}
Explain how to find the particular solution to each given IVP.
\begin{enumerate}[(a)]
\item \[ 0 = -{x''} - 81 \, {x} \hspace{2em}
{x} (0)= -3 ,
{x} '(0)= -45 \]
\item \[ -20 \, {y} + {y'} = -{y''} \hspace{2em}
{y} (0)= -3 ,
{y} '(0)= -12 \]
\end{enumerate}
\end{exerciseStatement}
\begin{exerciseAnswer}
\[ {x} = -3 \, \cos\left(9 \, t\right) - 5 \, \sin\left(9 \, t\right) \]\[ {y} = -3 \, e^{\left(4 \, t\right)} \]
\end{exerciseAnswer}