The energy of an electromagnetic radiation is $3\times {10}^{-12}\mathrm{ergs}$ . What is its wavelength in nanometers?  $\left(\mathrm{h}=6.625\times {10}^{-27}\mathrm{erg}.\sec ,\mathrm{C}=3\times {10}^{10}\mathrm{cm}.{\sec }^{-1}\right)$