A proton moves perpendicularly to a uniform magnetic ?eld B with a speed of 1.9 × 10 7 m/s and experiences an acceleration of 8 × 10 12 m/s 2 in the positive x direction when its velocity is in the positive z direction. The mass of a proton is 1.673 × 10 -27 kg. Find the magnitude of the ?eld.