Stopping potential is given by eVo=hcλ−ϕ
where
h: Planck's constant, c: speed of light, λ: wavelength, ϕ: work function
Hence,
eV=hcλ1−ϕ...........(i)
3eV=hcλ2−ϕ...............(ii)
From (i) and (ii),
ϕ=3hc2eλ1−hc2eλ2.........(iii)
When photons of wavelength λ3 is incident, stopping potential is equal to
eV′=hcλ3−ϕ.........(iv)
From (iii) and (iv),
V′=−3hc2eλ1+hc2eλ2+hceλ3