Question

Expression for de broglie wavelength of electrons in terms of electric potential

Answer 1

The problem with your first attempt is your assumption that:

f=cλ

This is true for light, and in fact it's true for any wave as long as we replace c by the phase velocity of the wave. The trouble is that the velocity you're calculating using the kinetic energy is the group velocity not the phase velocity, so your equation can't be applied to the electron.