Treating the box as a particle, there are two forces acting on it in opposite directions: the box's weight mg pulling it downward and the resistive force of the air pointing upward.
The box is falling at a constant speed, so by Newton's second law we have
mg - r = 0
where mg = (75.0 kg) (9.80 m/s²) = 735 N is the weight of the box and r is the magnitude of air resistance. So we have
r = mg = 735 N
