You are standing at x = 8.5 km. Lightning bolt 1 at x=0 km and lightning bolt 2 at x = 13.0 km. Both flashes reach your eye at the same time. Your assistant is standing at x = 3.0 km. If your assistant didn't see the flashes at the same time, what is the time difference between the two flashes? Express your answer using two significant figures and in microseconds.