PHYS GR6038 Lecture 4: Relativistic Corrections to the hydrogen-like atoms and heavier Feb6

I know this is a question about relativistic momentum but im not sure which equation to use. It seems like the kinetic energy equation can be copmined with the relativistic momentum equation but im not sure what to put in for mass.

Find the ratio of speeds of an electron and a negative hydrogen ion (one having an extra electron) accelerated through the same voltage, assuming non-relativistic final speeds. Take the mass of hydrogen ion to be . 

Find the ratio of speeds of an electron and a negative hydrogen ion (one having an extra electron) accelerated through the same voltage, assuming non-relativistic final speeds. Take the mass of the hydrogen ion to be .