Suppose the density of the Earth was somehow reduced from its actual value to 1300 kg/m3. What would be the value of g, the acceleration due to gravity, on this new planet? Assume the radius does not change.

I know that density = mass/volume & the volume of a sphere = 4/3pir^3
... I assumed after the volume was found I could use g= GM/r^2

maybe my algebra is wrong but I can't seem to get the right answer