The SI unit of Mass is the Kg. The mass of a body is constant. Force is calculated by Mass x Acceleration and the SI unit of force is the Newton (N). The force of gravity can change.
The mass of an aircraft is acted on by the acceleration due to gravity. In this question you are told the value of acceleration due to gravity to use. (10 meters per second squared.) 500 kg x 10 = 5000 N. This force acts at a distance of 16 m. 5000 x 16 = 80000 Nm
Make sure you use the correct units in the exam.