Equations of motion?
Without seeing the Q I'm fumbling in the dark, but I'm imagining the electrons enter some kind of electric field which is at right angles to their direction of motion?
If that's the case, the you will need to use the horizontal speed to work out how long they will be between the plates, then use this time, plus the acceleration, to work out what their upwards/vertical velocity will increase to in this time.
Remember it's like projectile motion so horizontal and vertical motion are completely independent which means that, assuming they enter the plates horizontally, their vertical velocity at that time is zero.
Of course, your Q might be nothing like that, but it's my best guess until I see it....