When a proton is released from rest in a room, it starts with an initial acceleration a0towards west. When it is projected towards north with a speed υ0it moves with an initial acceleration 3a0 toward west. The electric and magnetic fields in the room are

when proton is stationary force is only due to electric field  i.e, E⇀ =ma0e along westforce due to magnetic field must be   2ma0along west  its velocity is along north (j) force is along  west (-i)F→=q(V ⇀ xB⇀)    (-i=j x-k)  hence magnetic field is  vertically downwards direction and having a magnitude 2ma0e