Astudent measures the mass of an 8cm block of brown sugar to be 12.9g. what is the density of the brown sugar

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