I believe the question is missing this sentence: how fast should the bat fly in order to hear a beat frequency of 8.00 Hz? The solution would be like this for this specific problem:
8.00 / 1700 = 2 * v /
340m/s
v = 0.8 m/s
So, in order to hear a beat frequency of 8.00 Hz, the bat should fly 0.8m/s.