The mass of a block of stone is 6400 kg. if the block has a volume of 0.8 m cubed, what is its density? remember, density is mass per unit of volume.

due in less than 12 hours.

The question is asking for mass/unit of volume. 
Therefore the density of the material would be 6400kg/0.8m^3. You then want to change the bottom into a 1 by multiplying the top and bottom both by 10/8. You now have 8000kg/m^3, which is the solution. 

(-14+3/2b) - (1+8/2b) = -5 -5/2b(-10+b) + (7b-5) = 8b-15(7/2b-3) - (8+6b) = -5/2b-11(5+2b) + (2b+3/2) = 4b+ 13/2