Given:
Initial distance between the two planets is 3 AU
Final distance between the two planets is 1 AU
Required: Change in gravitational force.
Step-by-step explanation:
The relation between gravitational force and distance is

The initial gravitational force is

The final gravitational force is

As the force is inversely proportional to the distance squared.
Thus, the force is increased by 9x.
Final Answer: The gravitational force increased by 9x