First of all the equation you need is this one
E=Ek+ϕ
Ek=eV0
Please remember that the energy of a photon of wavelength lambda is given by E=hcλ
you can then calculate the maximum kinetic energy since you know the work function of the metal (1.8eV), and the wavelength of the photons (400 nm) (remembering to convert to SI units of joules for the work function). You can then find the maximum velocity of the electrons easy enough.You can obtain the stopping potential usign the next formula
Ek=eV0
Because it is a stopping voltage, V_{0} and remember it will have the units of volts.
I think with that I can help you a lot.
