A coordinate system (in meters) is constructed on the surface of a pool table, and three objects are placed on the table as follows: a 8.0 kg object at the origin of the coordinate system, a 16.0 kg object at (0, 2.0), and a 18.0 kg object at (4.0, 0). Find the resultant gravitational force exerted by the other two objects on the object at the origin. What is the direction?