# What is the density of a 91 g rock with a volume of 28.5 cm^3?

Density, $\rho$, is mass per unit volume.
Density of your rock: $\frac{91 \cdot g}{28.5 \cdot c {m}^{3}}$ $\approx$ $3 \cdot g \cdot c {m}^{- 3}$. You can do a better estimate than this.
Note that the units, $g \cdot c {m}^{- 3}$, are quite sensible and may be derived from the way the density is calculated..