Assume the downward direction is positive and upward direction is negative
W = weight of the object = 12 N
f = air resistance on the object = - 3 N
F = net force on the object
net force is the sum of forces acting on the object , hence the net forces is given as
net force = sum of forces
F = W + f
F = 12 + (- 3)
F = 12 - 3
F = 9 N
hence the size of net force on the object comes out to be 9 N
