ok so you have to know the formula density=mass/volume (density is the measure of how much mass is in each unit of volume)
the mass, as measured is 12.9 g
for the volume, to find the volume of the cube multiply the length times the width times the height. Or , since it's a cube, just cube 8
V=8*8*8=512 cm^3
therefore, density= 12.9 g/512 cm^3
since you need to have 1 cm^3 on the bottom, this simplifies to about
.025 g/cm^3
