An infinite line charge produces a field of 7.18 x 10⁸ N/C at a distance of 2 cm. The linear  charge density is

Given,  $E=7.18\times {10}^8 \mathrm{N}/\mathrm{C}$

and   $r=2 \mathrm{cm}=2\times {10}^{-2}$

Electric field is given by 

$E=\frac{\lambda }{2\pi {\epsilon }_{0}r}$

$\lambda =2\pi {\epsilon }_{0}rE$

$\lambda =\frac{4\pi {\epsilon }_{0}rE}{2}=\frac{2\times {10}^{-2}\times 7.18\times {10}^8}{2\times 9\times {10}^9}$

$=7.98\times {10}^{-4}\mathrm{C}/\mathrm{m}$