By definition, the gravitational force of two objects is given by:

Where,
G: gravitational constant
m1: object number 1
m2: object number 2
r: distance between objects
Suppose that one of the masses is twice its initial value.
We have then:

Rewriting the equation we have:

Substituting values:

Answer:
the gravitational force between the objects if the mass of one object is doubled is:
4800 N