In a certain region, the earth's magnetic field has a magnitude of 4.7 × 10-5 T and is directed north at an angle of 69° below the horizontal. An electrically charged bullet is fired north and 13° above the horizontal, with a speed of 500 m/s. The magnetic force on the bullet is 3.5 × 10-10 N, directed due east. Determine the bullet's electric charge, including its algebraic sign (+ or -).