Objects sliding down a rough plane...

I have calculated an answer (D), but am not totally sure about its accuracy. Please could you work out the correct answer so I can compare. (See attached file for full problem description)

Equation of motion of two springs

I think the answer is A but could you confirm (or correct) my calculations. (See attached file for full problem description)

Energy stored in springs

What is the energy stored in the two springs? (See attached file for full problem description)

Forming relationships between variables

Please help! I cannot seem to get this problem solved! (See attached file for full problem description)

Flight Simulation

A plane flying with a constant speed of 24 km/min passes over a ground radar station at an altitude of 9 km and climbs at an angle of 40 degrees. At what rate is the distance from the plane to the radar station increasing 3 minutes later? (The answer to this problem is not 14.375.)