The equation for finding the mass of a system is:
mass = density x volume
where density is the mass per unit volume of a substance, and volume is the amount of space that a substance occupies.
To use this equation, you need to know the density and volume of the substance in question. For example, if you have a block of iron with a density of 7.8 g/cm^3 and a volume of 50 cm^3, you can find its mass using the equation above:
mass = density x volume
= 7.8 g/cm^3 x 50 cm^3
= 390 g
This means that the mass of the block of iron is 390 grams.