A is the largest ellipse, so the numerator with a 2 as the denominator is X 2, and the focus is on the X axis. If the molecule is y 2, the focus is on the y axis. (that is, the denominator is x 2, the focus is on the x axis, and so is the y axis).
The a of the ellipse is not necessarily on the x axis. An ellipse has a long axis and a short axis, and A is generally on the long axis.
If you don't understand, you can ask, please adopt.