Charged particle within an electric field

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Homework Statement

Homework Equations

DON'T KNOW

The Attempt at a Solution

I can honestly say I've never been this confused by a question. At first my mind automatically went to a=qE/m since I have both charge and mass, but it became apparent that the equation was irrelevant because the velocity is constant. I thought it might be possible that I'm supposed to obtain the electric field by taking the integral of the electric potential, but I don't understand how I can get that using the information I have, and I have no sweet clue why the magnitude of the magnetic field is included.

 

[collinsmark]

It means the magnetic force exactly cancels the electric force (plus the gravitational force). What's the magnetic force on the ion?