I've already figured part a by setting potential energy (equalling charge multiplied by potential difference) equal to kinetic energy. My answer is 6.49*10^7 m/s.

However, b is where I'm having trouble. I don't know how to go about it. Could someone please point me in the right direction?

Since each electron has a charge of its own, the total charge for n number of electrons is the n times the charge of the electron
that is [tex] Q = Ne [/tex]