Because of lack of additional information, let us assume that our horizontal ellipse is centered at the origin. We know that the equation of a horizontal ellipse centered at the the origin is given by:

https://tex.z-dn.net/?f=+%5Cfrac%7Bx%5E2%7D%7Ba%5E2%7D+%2B%5Cfrac%7By%5E2%7D%7Bb%5E2%7D%3D1+%C2%A0

Such that a>b (condition for a horizontal ellipse)

Where a=\frac{1}{2}(major axis)=\frac{1}{2}\times 50=25

Likewise, b=\frac{1}{2}(minor axis)=\frac{1}{2}\times 20=10

Thus, the equation of our horizontal ellipse will be:

\frac{x^2}{25^2} +\frac{y^2}{10^2}=1

Please find the attached file for the graph of our horizontal ellipse.