Answer:
F1 = G m1 m2 / R^2 force between m1 and m2
F2 = G m1 m2 / (6 R)^2 force if distance increases by factor of 6
F1 / F2 = 6^2 = 36
One of the masses would have to be decreased by a factor of 36
for the force to remain the same - if either m1 or m2 were decreased to 1/36 of its value, then the force would remain the same