In this problem, you have a circle E with diameter CD, and radius EA. The line AB is tangent to the circle at point A. We're given AC = 28 and AD = 15, and we need to find CD.
First, let's draw a diagram:
```
C
|\
| \
| \
| \ E
| \
| \
15 | \
| \
| \
|_________\
A B D
```
Now, let's use the information we have:
1. AC = 28
2. AD = 15
Since AC is the diameter of the circle, and AD is a radius, we know that CD (the other radius) is also 28 (twice the length of AD).
Now, we have CD = 28, and we can round it to the nearest tenth if necessary. In this case, there's no rounding needed because 28 is a whole number.
So, CD = 28.