The light rays having photons of energy 4.2 eV are falling on a metal surface having a work function of 2.2 eV. The stopping potential of the surface is

According to Einstien, the photoelectric equation hv=KE_max−W=eV_s−W.
Where KE_max=eV_s= maximum KE of the photoelectrons in electron volt and V_s is the stopping potential. W is the work function of the material. hv is the energy of the incident photons.
Here, hv =4.2eV
W=2.2eV 
So, eV_s =hv−W=4.2−2.2=2eV  
V_s = 2 V 
So, here stopping potential is 2 V.