Displacement verctors of 1km south, 3km north, 6 km south, and 2 km north combine to a total displacement of

                              
                    (1km south) + (3km north) + (6 km south) + (2 km north)

             =      (1km south + 6km south)  +  (3km north) + (2km north)

             =                (1km + 6km) south  +  (3km + 2km) north

             =                     7km          south  +          5km       north

             =                                        2 km south   .        

This is the answer: body 1 + engine 2