In this lesson, students make sense of subtracting signed numbers by representing it on the number line. First, they rewrite subtraction equations as addition equations with a unknown addend—for example, can be thought of as . Students represent these equations on a number line and find the unknown value—to get from 3 to -8, the second arrow must be 11 units long and pointing to the left, so . From several such examples, students generalize that subtracting a number gets the same value as adding its opposite—in this case, and are both equal to -11. As students use the results from multiple subtraction expressions to notice a general pattern, they are making use of repeated reasoning (MP8).