How do i work out a passenger on an airplane's kinetic energy relative to the airplane?? please help!!

The kinetic energy of the passenger is one half their mass times their speed relative to the plane squared. The kinetic energy of the plane would be zero in the moving frame of reference of the plane as long as it doesn't accelerate.

For example, if the person weighs 40 kg, and moves 0.5 meter per second to the back or front of the plane, their kinetic energy relative to the plane would be
1/2 (40 kg)\*(0.5 m/s)^2 = 20 kg \* 0.25 (m/s)^2

This would give them a kinetic energy of 5 Joules.

Hope this helps.