An ellipse has always a major axis and a minor axis. And the foci lie on major axis.

In your question, the major axis is y-axis and its length is 6. The minor axis is x-axis and its length is 4.

Formula for eccentricity:

where b is length of minor axis

and, a is length of major axis

And the focus: where a is the length of major axis.

The formula usually given in textbooks assume that a is the major axis whereas in your question a is the minor axis...hence we have to swap a and b