check the picture below.
so, if the semi-circle has a diameter of 36, then the radius is half that, or 18
[tex]\bf \textit{area of a semi-circle}\\\\
A=\cfrac{\pi r^2}{2}\quad
\begin{cases}
r=radius\\
-----\\
r=18
\end{cases}\implies A=\cfrac{\pi 18^2}{2}\implies A=\cfrac{324\pi }{2}
\\\\\\
A=162\pi [/tex]
