"two planets have the same mass, but planet a has 3 times the radius of planet b. how do the surface gravities of the two planets compare? "

The gravitational acceleration on the surface of planet A is:

where G is the gravitational constant,  is the mass of planet A and  its radius.

Similarly, the gravitational acceleration on the surface of planet B is:


The ratio between the gravitational acceleration on planet A and B becomes:


The problem says that the two masses are equal:  while planet A has 3 times the radius of planet B: . Substituting into the ratio, we get:


so, gravity on planet B is 9 times stronger than planet A.