Please help with a,b, and c. using college physics algebra show equations and all of work.

A tube is open on one end, closed on the other, and is used to make sound. On a -5.00 C day, the tube makes a first harmonic frequency of 305Hz. a. How long is the tube? b. When 6 identical versions of the tube are played they have a sound level of 75.0 dB. What is the sound level when only one tube is played? c. The tube travels at an unknown velocity, while an observer travels towards the tube at a speed of 20.0 m/s At what speed, and which direction, must the Tube be traveling for the observer to hear a frequency of 385 Hz made by the tube