Explain the differences in finding the density of a rectangular Solid object and finding the density of an irregular object

Mass/volume

for an irregular solid, the easiest way to determine volume is to fill a graduated cylinder to a certain level, record the level of water, then carefully place your solid in the cylinder and record the height of the water after the solid is in.

the volume of a regular solid by measuring the lengths of its sides and determining the volume by multiplying length x height x width