Answer
$\dfrac{x^2}{36}+\dfrac{y^2}{27}=1$
Work Step by Step
The standard form of the equation of the ellipse when the major axis is horizontal can be expressed as: $\dfrac{(x-h)^2}{a^2}+\dfrac{(y-k)^2}{b^2}=1$ in which $(h,k)$ is the center, $2a$ is the major axis length, and $2b$ is the minor axis length.
The standard form of the equation of the ellipse when the major axis is vertical can be expressed as: $\dfrac{(x-h)^2}{b^2}+\dfrac{(y-k)^2}{a^2}=1$ in which $(h,k)$ is the center, $2a$ is the major axis length, and $2b$ is the minor axis length.
Since the ellipse is horizontal, the distance between the vertices is equal to $2a$:
$b=\sqrt{a^2-c^2}=\sqrt {6^2-3^2}=\sqrt {27}$
$\dfrac{x^2}{a^2}+\dfrac{y^2}{b^2}=1$
or, $\dfrac{x^2}{36}+\dfrac{y^2}{27}=1$
