$a)\frac{{100}^o}{{360}^o}\cdot 2\pi \cdot 9\text{cm}=\frac{10}{{\sout{36}}^2}\cdot {\sout{18}}^1\pi =5\pi$  

$b)\frac{{270}^o}{{360}^o}\cdot 2\pi \cdot 8\text{cm}=\frac{3}{{\sout{4}}^1}\cdot {\sout{16}}^4\pi \text{cm}=12\pi \text{cm}$  

$c)\frac{{24}^o}{{360}^o}\cdot 2\pi \cdot 7\text{cm}=\frac{2}{3}\cdot 14\pi \text{cm}=\frac{28}{3}\pi \text{cm}=9\frac{1}{3}\pi \text{cm}$