Given:
Diameter =18
[tex]\begin{gathered} \text{radius}=\frac{D}{2} \\ =\frac{18}{2} \\ =9 \end{gathered}[/tex]Perimeter of semicircle is:
[tex]=\pi r+2r[/tex]So the perimeter is:
[tex]\begin{gathered} =\pi(9)+2(9) \\ =9\pi+18 \\ =28.2743+18 \\ =46.2743 \end{gathered}[/tex]