Answer: 99.1 seconds
Step-by-step explanation: You know that the total distance is 34km, and the speed of the sound is 343m/s.
Now, the total time that takes to something with a given velocity, to reach a distance D, is:
T = D/v
where D = 34km and v = 343m/s
Now, here we have different units, so we must convert the kilomeres into meters, this can be done as: 1km = 1000m then, 34km = 1000*34m = 34000m
Then the total time is:
T = 34000m/343m/s = 99.1 seconds