Lesson objective: Determine the length of an unknown side of a shape when the length of the other sides and perimeter are given.

This lesson helps to build fluency with understanding perimeter. Rectangles and other polygons are used here because perimeter applies to any polygon. This work develops students' understanding that perimeter is a linear measure of an attribute of a plane figure.

Students engage in Mathematical Practice 1 (make sense of problems and persevere in solving them) as they determine the unknown side of a figure by using what they know about perimeter.

**Key vocabulary:**

- length
- perimeter
- width