If you are working with the law of universal gravitation equation:
equation image indicator

what would happen to the magnitude of the force if the distance between the 2 masses is increased by 6?

It would be decreased by 1/36

Explanation:

According to Newton's law of Gravitation, the magnitude of the gravitational force exerted between two bodies of masses and  and separated by a distance  is equal to the product of their masses and inversely proportional to the square of the distance:  

  (1)  

Where is the Gravitational Constant

Now, if the distance between both bodies is , equation (1) becomes:

  (2)  

  (3)  

This means the magnitude of the gravitational force decreases by