Answer
$\frac{x^2}{36}+\frac{y^2}{9}=1$
Work Step by Step
If the vertex of an ellipse is at $(\pm a,0)$ and the co-vertex is at $(a,\pm b)$, then the equation of the ellipse is $\frac{x^2}{a^2}+\frac{y^2}{b^2}=1$
Hence here $a=-6,b=-3$, and the equation is: $\frac{x^2}{36}+\frac{y^2}{9}=1$
