In region `x gt 0` a uniform and constant magnetic field `vec(B)_(1) = 2 B_(0) hat(k)` exists. Another uniform and constant magnetic field `vec(B)_(2) = B_(0) hat(k)` exists in region `x lt 0` A positively charged particle of mass m and charge q is crossing origin at time t =0 with a velocity `bar(u) = u_(0) hat(i)`. The particle comes back to its initial position after a time : (`B_(0),u_(0)` are positive constants)