question:
http://i40.tinypic.com/2hwcswm.gif

solution:
$\displaystyle
\sigma=\frac{7.5\mu c}{14}=0.53571428571428571428571428571429\mu c\\
$
$\displaystyle
14=2\pi r\\
$
$\displaystyle
r=\frac{14}{\pi}\\
$
$\displaystyle
dq=\sigma r d\theta \\
$
$\displaystyle
k\sigma r\int_{0}^{\pi}d\theta=k\sigma r (\pi-0)=...
$

is it a correct solution?