Magnitude and direction of uniform magnetic field

[mwadhwa]

Homework Statement

A proton, with mass 1.67*10^-27 kg and charge +1.6*10^-19C, is sent with velocity 5.0*10^4 m/s in the x-direction into a region where there is uniform electric field of magnitude 390 V/m in the y direction. What is the magnitude and direction of the uniform magnetic field in the region, if the proton is to pass through undeflected? Assume that the magnetic field has no x-component. Neglect gravitational effects.

Homework Equations

The Attempt at a Solution

I have no idea where to even begin with this one! Any helps is greatly appreciated!

 

[ibnsos]

Think about controling velocity for a mass spectrometer. You know the E field and Velocity, how do they relate to the B field? (charge cancels in the balanced equation.)