so.. hmm notice the picture below
keep in mind that, the diameter is 24, radius is half that then, and
that the point of tangency, or, where the diameter, or radius, meets the tangent, is a right-angle
thus, just use pythagorean theorem to find the tangent's length
[tex]\bf c^2=a^2+b^2\implies \sqrt{c^2-a^2}=b\quad
\begin{cases}
c=\textit{distance from "a" to center}\\
a=\textit{radius length}\\
b=\textit{tangent line}
\end{cases}[/tex]
