Hi Saksham,

We know that kinetic energy is 1/2 mv^2

According to the question mass is equals to 9 * 10^-31  and the velocity is 6*10^5 .

Putting the values of mass and velocity in the formula of kinetic energy we will get 162*10^-21 i.e.  approximately 1 ev.

Now the energy of photon E is 12400/4000=  3.1 eV.

And E= kinetic energy+ fi

3.1 = 1+ fi

Fi = 2.1 eV

I hope your question has been answered properly.

All the best for your future!