In this lesson, students apply knowledge of the distributive property to find area. They relate earlier experiences decomposing arrays to using the distributive property to determine the unknown side length of a rectangle. For example, if a rectangle has an area of 72 square units, students might decompose the area into an 8 by 5 rectangle and an 8 by 4 rectangle. The sum of the side lengths, 5 + 4, gives the length of the unknown side.