I'm assuming you know some basics about ellipses in this solution:

For ellipse D:

An important equation for ellipses: where a = 5, b =3, e = eccentricity

Focus S is

For ellipse E:

Because b is larger than a, the major axis, where the focus lies, is the y axis.

where a = 2, b = 3

Focus T is

Now use the distance formula to find the distance between ST. Since there are two focus for each ellipse, I imagine you need to find the distance between each of them, but I'm pretty sure they will all be the same distance.