This Instructional Activity is a part of the Lesson 5, Unit 5, Grade 6. If students are struggling to see this relationship, it might be helpful for them to complete the rectangle that encloses the figure: How do we know which operation to use when finding missing side lengths? If we know two short sides (vertical or horizontal), we add to find the longer side. If we know the long side and one short side (vertical or horizontal), we subtract. These examples used whole numbers for the lengths of the sides.