If some unknown solid weighs 84.0 grams and occupies 30.0 cm3 of space, what is its density?

The density is 2.8 grams/cm ³.

The density of an object is the ratio between the mass of the object and the volume occupied by that mass of the object. Therefore, the formula:

density = mass / volume

density = 84 / 30

density = 2.8 g/cm ³