Please help me solve these problems. Show all steps. Thank you!

1. A bat flying toward a wall at 6.0m/s emits a sound at a frequency of 35kHz. What is the frequency of the echo (sound reflected by the wall back to the bat) that the bat hear?

2. . A sound from a speaker has a sound level of 54dB at a distance of 12m from the speaker. What is the sound level at 24m from the speaker assuming a spherical sound wave?

3. An open tube has a length of 1.20m. What are the two lowest frequencies of this tube at 31oC?