Magnetism

Force due to EF is in -ve z direction so it will reduce the velocity in z direction.
B is in z direction so a force will act along +ve x axis now let after time t the velocity vector be
V_t= Vi + Vj + Vk
Now the magnitude of Vi and Vj will remain equal to V_o as net velocity doesnt changes due to magnetic field
so we have reduce Vk to zero
V₀ (k) = (qE/m )*t
t= V₀m/qE