Answer:
        F = 8.6 10⁻¹² N
Explanation:
For this exercise we use the law of conservation of energy
Initial. Field energy with the electron at rest
          Em₀ = U = q ΔV
Final. Electron with velocity, just out of the electric field
          Emf = K = ½ m v²
           Em₀ = Emf
           e ΔV = ½ m v²
           v =√ 2 e ΔV / m
           v = √(2 1.6 10⁻¹⁹ 51400 / 9.1 10⁻³¹)
            v = √(1.8075 10¹⁶)
            v = 1,344 10⁸ m / s
Now we can use the equation of the magnetic force
          F = q v x B
 Since the speed and the magnetic field are perpendicular the force that
         F = e v B
         F = 1.6 10⁻¹⁹  1.344 10⁸ 0.4
        For this exercise we use the law of conservation of energy
Initial. Field energy with the electron at rest
          Emo = U = q DV
Final. Electron with velocity, just out of the electric field
          Emf = K = ½ m v2
           Emo = Emf
           .e DV = ½ m v2
           .v = RA 2 e DV / m
           .v = RA (2 1.6 10-19 51400 / 9.1 10-31)
            .v = RA (1.8075 10 16)
            .v = 1,344 108 m / s
Now we can use the equation of the magnetic force
          F = q v x B
 Since the speed and the magnetic field are perpendicular the force that
         F = e v B
        F = 1.6 10-19 1,344 108 0.4
        F = 8.6 10-12 N