In a photoelectric experiment using a sodium surface, you find a stopping potential of 1.85 V for

Question:

In a photoelectric experiment using a sodium surface, you find a stopping potential of 1.85 V for a wavelength of 300 nm and a stopping potential of 0.820 V for a wavelength of 400 nm. From these data find
(a) A value for the Planck constant,
(b) The work function ϕ for sodium,
(c) The cutoff wavelength l0 for sodium.