Consider the systems in Problem
(a) Determine and plot the equilibrium points and null-clines for the systems.
(b) Show the direction of the vector field between the null-clines as illustrated in Example 2 and Fig. 2.6.4.
(c) Sketch some solution curves starring near; but not on, the equilibrium point (s).
(d) Label each equilibrium as stable or unstable depending on the behavior of solutions that start nearby, and describe the long-term behavior of all the solutions.

dx/dt = y
dy/dt = x - 3y