The document explains how to calculate the mass of a piece of metal using Riemann sums. It defines volume as length times width times height, with width set to 1 cm. Density is defined as the square of x, where x is the average density. Mass is calculated as volume times density. A left-hand Riemann sum is used to calculate the mass by dividing the interval from 0 to 4 into 4 equal subdivisions and summing the areas of the rectangles.