Ahh! Finally!!!! I think I found another way!!!!
Here's my own drawing...it's not as accurate as CPhill's :
Each of the purple lines is a radius of the circle, " r ". Each radius meets the sides of the kite at right angles because each side of the kite is tangent to the circle.
Notice that there are two kites formed within the big kite. We can be sure that both these inside kites are similar to kite ABCD because all the corresponding angles are the same.
Looking at the kite ABCD to the little kite, we can say...
\(\frac{24}{18}\,=\,\frac{r}{18-r} \\~\\ 24(18-r)\,=\,18r \\~\\ 432-24r\,=\,18r \\~\\ 432\,=\,42r \\~\\ \frac{432}{42}\,=\,r \\~\\ r\,=\,\frac{72}{7}\qquad\text{cm}\) Cross multiply