the first step is making objective observations.
they have to be based on a certain event and can be confirmed by others if it is right or not.
correct answer is positive force → balanced force → negative force
when diego gives a big lift to the piano with the of pulley system that time, initially piano moves upward so positive force acts on it.
later, piano stops, in this condition balanced force act on it since force applied by diego is balanced by gravitational force of earth.
after some time piano starts falling in this condition negative force will act on the piano. in this condition, gravitational force will acts on piano in downward direction.