(a) The car pulls on the trailer (150 N to the left) therefore the trailer pulls on the car (150 N to the right).

(b) Fnet = 150 - 20 = 130 N (to the left)

Fnet = ma => 130 = (190)a => a = 13/19 m/s^2

(c) Constant speed and unchanging direction => a = 0. Fnet = ma => Fnet = 0 N. (This would mean that the resisitive force on the trailer has increased to 150 N).