The work function for Tungsten and Sodium are 4.2 eV and 2.3 eV, respectively. If the threshold wavelength for Sodium is 5460 Å, then that for tungsten is:

2990 Å

Threshold wavelength is given by $\lambda_0 = \frac{hc}{\phi}$

Where $\phi$ is work function of the material.

$\lambda_{01} = \frac{hc}{\phi_1}$

$\lambda_{02} = \frac{hc}{\phi_2}$

$ \frac{\lambda_{02}}{\lambda_{01}} = \frac{\phi_1}{\phi_2} = \frac{2.3}{4.2}=0.547$

$ \lambda_{02} = 0.547\times 5460 = 2990A^o$