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.