Step 1 :
It is given that
Foci is $( \pm 3, 0)$ and a = 4.
Since the foci lies on the x - axis, the major axis should be along x - axis.
Hence the equation of ellipse should be of the form
$c^2=a^2-b^2$ or $b^2=a^2-c^2$
Now substituting the values for c and a we get,
$ = 7$
Hence the equation of the ellipse is