Solve B please: (please take D on BC,there's a printing error)

Dear Student, 
Solution) As, the corresponding sides of similar triangles $∆ADB~∆CDA(already proved in A part)$ are proportional. 
$$\begin{gathered} \frac{CD}{AD}=\frac{DA}{DB} \\ A{D}^2=CD\times DB=18\times 8=144 \\ AD=12cm \\ Regards! \end{gathered}$$