Chapter 12.6, Problem 14E

In Problems $13-18$ , show that the given system or equation has only constant periodic solutions in the specified domain D.

$\begin{array}{rll}\frac{dx}{dt}& =& 2x-y-x{y}^2,\\ \frac{dy}{dt}& =& x-3y-x^{2}y\end{array}$

(D is the entire plane.)