The document describes a problem where a triangle with sides of 10, 17, and 21 units has a square inscribed inside it such that one side of the square lies on the longest side of the triangle. It uses Heron's formula to calculate the area of the triangle as 84 square units. It then sets up a ratio between the similar triangles formed by the altitude of the triangle and square to find that the length of the side of the inscribed square is 168/29 units.