Here is the correct way to handle what is called "reversed differences", in this caseand . You agree that and are not the same quantity, right? Now you got that far. But you cannot cancel and . They are DIFFERENT quantities, just as and are different quantities. Here is the way to handle it. Write the second term first and the first term second, that is, write as . Then you have Now factor out of , like this . When you factor out -1 it changes both signs inside: Now you can cancel the 's and you end up with or Edwin