# Phase plane portrait center ellipse equations

If I have a system of differential equations, with coefficient matrix, A=$$\begin{bmatrix} 1&13\\-2&-1\end{bmatrix}$$. The eigenvalues are $$\lambda= \pm 5i$$. The phase portrait is a center with a clockwise direction field. How would I go about determining the equation of one of the rotated ellipse orbits?

Equation for the phase paths: $$\frac {dx}{dy}=\frac {x+13y}{-2x-y}$$ $$({-2x-y}){dx}= ({x+13y})dy$$ $$-2x{dx}= ydx+xdy+13ydy$$ Note that $$ydx+xdy=dxy$$ : $$-2x{dx}= dxy+13ydy$$ Integrate: $$C=x^2+ xy+ \frac {13} 2 y^2$$ It's an ellipse.