[tex]\bf \textit{circumference of a circle}\\\\
C=2\pi r\quad
\begin{cases}
r=radius\\
-----\\
C=12\pi
\end{cases}\implies 12\pi =2\pi r\implies \cfrac{12\pi }{2\pi }=r\implies 6=r\\\\
-------------------------------\\\\
\textit{total surface of a cylinder}\\\\
S=2\pi r(h+r)\quad
\begin{cases}
r=6\\
h=\stackrel{half~of~r}{3}
\end{cases}\implies S= 2\pi (6)(3+6)[/tex]
and surely you know how much that is.
