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

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

is it a correct solution?