A sodium lamp gives out yellow light of wavelength 589 nm…

Step 1. Frequency. $\nu = \dfrac{3.00 \times 10^{8}}{589 \times 10^{-9}} = 5.09 \times 10^{14}\ \text{Hz}$.

Step 2. Wavenumber. $\tilde{\nu} = \dfrac{1}{589 \times 10^{-9}} = 1.698 \times 10^{6}\ \text{m}^{-1}$.

Step 3. In cm. $1.698 \times 10^{6} \times 0.01 = 16\,980\ \text{cm}^{-1}$.