A parallel plate capacitor is assembled using two square plates with a side length of 5.0 cm and a spacing of 1.0 mm between the plates. The electric field strength between the plates is 4.0 × 10⁷ V/m. If a proton is shot from the negative plate towards the positive plate and arrives moving at 3.2 × 10⁶ m/s, what was the proton's speed as it left the negative plate?