Lesson objective: Connect the ideas of using a letter to stand for an unknown value and writing area/perimeter equations.
This lesson provides an opportunity for students to apply their knowledge and understanding of using letters to represent an unknown value to a mathematical situation. Students are asked to develop equations for finding the area and perimeter of a rectangular figure and apply those to writing equations and solving for unknowns in area and perimeter word problems.
Key Concept students will use:

We can represent problems using equations with letters standing for the unknown quantity.
Skills students will use:
 Developing strategies for addition and subtraction Part 2 (Grade 3, Unit 3)
 Solve problems involving area (Grade 3, Unit 12)
 Represent unknown quantities in equations using letters (Grade 3, Unit 15)
Students engage in Mathematical Practice 2 (reason abstractly and quantitatively) as they make sense of quantities and their relationships. Students should be comfortable in understanding their quantities and unknowns; not just computing them while being flexible in their use of the operations and properties. Teachers should have students communicating the meaning of the quantities, unknowns, and symbols in their problems, as well as the meaning of their solutions.
Key vocabulary:
 area
 equation
 perimeter
Special materials needed:
 graph paper
 ruler
 string