The difference between threshold wavelengths for two metal surfaces A and B having work  function ${\varphi }_A=9 eV$    and  ${\varphi }_B=4.5 eV$   in nm is (Given, hc = 1246 eV-nm)

Given,  ${\varphi }_A=9eV$  ${\varphi }_B=45eV$
Threshold wavelength of metal surface, A
${\lambda }_A=\frac{1242}{{\varphi }_A}nm=\frac{1242}{9}nm$  
Similarly,  ${\lambda }_B=\frac{1242}{4.5}$   ${\lambda }_B-{\lambda }_A\frac{1240}{4.5}=\frac{1240}{9}$
                    
                                = 275.55 – 137.77
                                = 137.78 – 138 nm