In this case, the first point we have to remember is that we do not get a single value when we add two or more terms which are unlike in nature. This certainly obviates the need for us to look at the result when we add or subtract unlike terms. That is, addition of -7abc and 3bc will be either -7abc + 3bc or 3bc - 7abc and the result if we wish to subtract these terms will be either -7abc - 3bc or 3bc + 7abc depending on whether we consider either 3bc or -7abc as the first or the second term.