How would I sketch the ellipsoids $\displaystyle x^t \Sigma^{-1} x <= 1$ for

$\displaystyle \Sigma =\begin{bmatrix}5 &4\\ 4 &5\end{bmatrix}$ and $\displaystyle \Sigma=\begin{bmatrix}3 & 0\\ 0 &3\end{bmatrix}$

clearly indicating the principal axes of the ellipsoid and the intersection of the ellipoids with the axes.

I know that the equation for an ellipsoid $\displaystyle \frac{x^2}{a} + \frac{y^2}{b} + \frac{z^2}{c} = 1$ but not sure how or if it even applies in this problem.

Any help would be greatly appreciated.