A proton initially moves left to right long the x-axis at a speed of 2 ´ 103 m/s. It moves into an electric field, which points in the negative ´ direction, and travels a distance of 0.2 m before coming to rest. If the proton's mass and charge are 1.67 ´ 10-27 kg and 1.6 ´ 10-19 C respectively, what is the magnitude of the electric field?