A 150-V battery is connected across the two parallel metal plates of area 28.5 cm² and separation 8.20mm. A beam of alpha particles (charge +2e, mass 6.64x10^-27kg) is accelerated from rest through a potential difference of 1.75kV and enters the region between the plates perpendicular to the electric field, as shown in the figure ( battery hooked up to parallel plates [capacitor]) What is the direction and magnitude of the magnetic field field so that the alpha particles emerge undeflected from between the plates.
Diagram in textbook is like this but rotated countclockwise 90 degrees
I know that Fe and Fm are equal but opposite, so B must be out of the page and velocity = E/B for a charge going through an electric field/magnetic field undeflected. I'm kinda stuck after that.