We first need to define our direction in our system. The tugboat is moving eastward. So on an x and y axis we can assume the tugboat is moving in the positive (+) x direction. The barge is moving westward. So on an x and y axis we can assume the barge is moving in the negative (-) x direction.
(P)system = (m1)(v1) + (m2)(v2)
(P)system = (15000kg)(+6.0m/s) + (1.2x10^5kg)(-1.2m/s)
(P)system = -54000
The momentum of the system favors moving westward.
