I need help solving these problems. Please show all of the steps to each solution! Thank you.

1. A string has a length of 0.540m. This string has a linear density of 0.050kg/m, and it is under a tension of 120N. What is the wavelength and frequency of the 2nd harmonic of this string?

2. A closed tube has a length of 0.75m. What are two lowest frequencies of this tube at 25oC?

3. One jet engine produces a sound level of 120dB. What is the sound level of four identical jet engines?

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

5. A car moving at 18m/s is approached from behind by a police car moving at 26m/s. If the police siren emits a sound at 520Hz, what is the frequency heard by the driver of the car?