You may want to review equations of circles first.
Equations of ellipses look like x2 / 36 + y2 / 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 x2 + 4y2 − 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 x2 / 64 + y2 / 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.