Resultant force is zero when something moves at a constant speed because both forces are equal to each other. For example a skydivers mass (due to gravity) becomes equal to the air resistance, so the resultant force is zero
this is becasue all the force on the object is balanced meaning that the object is either stationary or moving at a constant speed. If it is moving at a constant speed then that means that the acceleration is zero because all the forces are balanced. Therefore the resultant force is zero if acceleration is zero because newtons first law states that if:
mass X acceleration = 0 then resultant force is zero
75kg x 0 = 0 meaning the object is at concstant speed. U know this because if it was not constant then the onbject would be accelerating because the forces wuld not be balanced!!