A nucleus has a radius of 4 Fermis.
Use the uncertainty principle to estimate the kinetic energy for a neutron localized inside the nucleus.
Do the same for an electron.
Try non-relativistic formula first and verify approximation when we have the energy.
This is much less than 940 MeV so the non-relativistic approximation is very good.
The electron energy will be higher and its rest mass is only 0.51 MeV so it WILL be relativistic.
This makes it easier.