Answer
So work function of the metal is $W_{0}\approx2.56eV$
Work Step by Step
Given that The maximum wavelength that can eject electron from the surface of the metal is $\lambda=485nm=485\times10^{-9}m=4.85\times10^{-7}m$
$E=\frac{hc}{\lambda}$ tells us that if the wavelength is maximum the energy will be minimum. So above wavelength corresponds to minimum energy required to eject an electron from the surface of the metal , which is called work function .
so work function $W_{0}=E=\frac{hc}{\lambda}$
putting $h=6.63\times10^{-34}J.s$, $\lambda=4.85\times10^{-7}m$,, $c=3\times10^{8}m/s$
$W_{0}=\frac{6.63\times10^{-34}J.s\times3\times10^{8}m/s}{4.85\times10^{-7}m}=4.1010\times10^{-19}J$
Now since $1.6\times10^{-19} J$ is equal to $1eV$
$1J$ is equal to $\frac{1}{1.6\times10^{-19}}eV$
so $4.1010\times10^{-19} J$ is equal to $\frac{4.1010\times10^{-19}}{1.6\times10^{-19}}eV$= $2.5631eV$
so
$W_{0}=4.1010\times10^{-19}J=2.5631eV\approx2.56eV$
