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