Consider the system of differential equations
$$
\begin{aligned}
& \dot{x}=4 \beta x-y \\
& \dot{y}=9 x+\beta y
\end{aligned}
$$
where $\beta$ is a real-valued parameter.
(a) Solve the system for arbitrary initial conditions and $\beta=0$.
(b) Find two critical values of the parameter, $\beta_1<0$ and $\beta_2>0$, at which the nature of the solution changes. Discuss the solutions for $\beta=\beta_1$ and $\beta=\beta_2$.
(c) Draw phase portraits which describe qualitatively the nature of the solutions for $\beta<\beta_1, \beta_1<\beta<\beta_2$, and $\beta>\beta_2$.