Why is an objects density expressed as a combination of two units

An objects density is expressed as a combination of two units because the equation for density is Mass/Volume=Density. Therefore, both the mass and the volume retain a unit measure and when they are put together they create an expression that is a combination of two units such as kg/m^3 or g/cm^3. 

The purpose of a lab is to do your "expirements" the importance of the topic is so you can now what the story is about and the last one is because no one is me with my question ( what is 3 over 4 x - 9 = 27 maybe you can )