You may want to review equations of circles first.

Equations of ellipses look like *x*^{2} / 36
+ *y*^{2} / 25 − 1 = 0. The center of this
ellipse is at the origin, as it always will be in this section.
What makes an ellipse different from a circle is that it
has *two* radii (the plural of radius), one horizontal and
one vertical. This is because an ellipse is like an oval; it's
longer in one direction than in the other.

To read off the radii from the equation, you take the square roots of the denominators. So the horizontal radius is 6 (the square root of 36) and the vertical radius is 5 (the square root of 5).

Sometimes, the equation will be given to you
like *x*^{2} + 4*y*^{2} − 64 = 0.
You can turn that into the usual form by dividing both sides by
the constant, which in this case is 64. In this case, that will
turn it into *x*^{2} / 64 + *y*^{2} / 16
− 1 = 0, so the horizontal radius is 8 and the vertical
radius is 4.

Of course, if the horizontal and vertical radii are the same, then our ellipse is really just a circle.