Two solid spheres of radius R made of the same type of steel are placed in contact. The magnitude of the gravitational force they exert on each other is F1. When two other solid spheres of radius 3R made of this steel are placed in contact, what is the magnitude of the grav force that they exert on each other?
a) F1
b) 3F1
c) 9F1
d) 81F1

I know the correct answer is (D) but I can't figure out how...I initially thought that using the formula for grav force it would just be R^2 (or 3^2) so (C), but that's incorrect. Can anyone explain?

Question

Two solid spheres of radius R made of the same type of steel are placed in contact. The magnitude of the gravitational force they exert on each other is F1. When two other solid spheres of radius 3R made of this steel are placed in contact, what is the magnitude of the grav force that they exert on each other?
a) F1
b) 3F1
c) 9F1
d) 81F1

I know the correct answer is (D) but I can't figure out how...I initially thought that using the formula for grav force it would just be R^2 (or 3^2) so (C), but that's incorrect. Can anyone explain?