Answer
$\frac{x^2}{100}+\frac{y^2}{36}=1$
Work Step by Step
The ellipse has a major axis parallel to the x-axis, hence the equation of the ellipse here has the form $\frac{(x-h)^2}{a^2}+\frac{(y-k)^2}{b^2}=1$, where $a\gt b\gt0$ and $(h,k)$ is the center.
Assume $(h,k)=(0,0)$. $a=\frac{20}{2}=10$, $b=6$, hence the equation is: $\frac{(x-(-0))^2}{10^2}+\frac{(y-0)^2}{6^2}=1\\\frac{x^2}{100}+\frac{y^2}{36}=1$
