What is the work function of the metal if the light of wavelength $4000\mathrm{\AA }$ generates photoelectrons of velocity $6\times {10}^5{\mathrm{ms}}^{-1}$ from it?

(Mass of electron $\mathrm{ }=9\times {10}^{-31} \mathrm{kg}$, velocity of light $=3\times {10}^8 {\mathrm{ms}}^{-1}$, Planck's constant $=6.626\times {10}^{-34} \mathrm{Js}$, Charge of electron $=6.626\times {10}^{-34} \mathrm{Js}$)