In this lesson, students build fluency with subtracting signed numbers. They learn that “the difference of and ” means . They see that the difference between two numbers can be positive or negative depending on the relative position of the numbers on the number line. For example, , while . If students had previously understood subtraction to involve always subtracting the lesser number from the greater number, then this lesson is an opportunity for them to develop a more nuanced understanding of subtraction. As students write subtraction expressions to represent differences in elevation, they are reasoning abstractly and quantitatively (MP2).