I know the answers to these questions:
(a) How does quadrupling (i.e. increasing by 4 times) the distance between two objects a ect the
gravitational force between them?
Answer: Strength between two objects decreases with the square of the distance between their centers The gravitational force (Fg) follows an inverse square law. By quadrupling the distance between two objects the gravitational force (Fg) will decrease by :
1/4^2 = 4^2= 16.

(b) Suppose the sun were somehow replaced by a star with twice as much mass. What would happen to the gravitational force between the Earth and the Sun?
Answer: The strength of the gravitational force (Fg) is directly proportional to the product of their masses. They increase in linear fashion so gravitational force (Fg) will be twice as strong if one of the masses increases by a factor of 2.

(c) Suppose the Earth were moved to one-third of its current distance from the Sun. What would happen to the gravitational force between the Earth and the Sun?
Answer: 1/3^2 = 3^2=9. The strength of gravity between two objects decreases with the square of the distance between the centers. Hence, if the earth moved to 1/3 of its current distance from the sun, the force of gravity will be 9 times stronger than it is at this point in time.