hi

 

the velocity of the charge is given in +ve x direction & magnetic field is into the plane therefore by fleming's left hand rule the force on the charge due to Magnetic field will act in +ve y direction.

& since magnetic field always acts perpendicular to the velocity of charge hence the charge will have circular motion now centripetal force = force on charge due to magnetic field

(mv^2)/R = qvB

R = mv/qB

but  = q/m => q = m

=> R = v/B

now y = diameter of circle

therefore y = 2v/B