Question 737121
{{{-c}}} is another way of expressing {{{(-1)*c}}}.
A basic law of real numbers is  {{{c+(-c)=0}}}.


For two real numbers a and b, a-b means the same thing as a+ (-b).


Review those facts for a few minutes and try.... understand them.


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{{{(a)(b)+(-a)(b)+(b)(-a)-(a)(b)-(-a)(-b)=ab-ab-ab-ab-ab}}}, can you see how?
={{{ab+(-ab)+(-ab)+(-ab)+(-ab)}}}, but do you know why I show it this way?
=.
={{{-3ab}}}, Excuse me for skipping a step or two.


post-note:  There are a couple of ways to go from the step just before the "=." line.  The more pathways are shown, the more confusing a solution can be for a student.  What may be best if a step is missing is for the student to try to fill in any missing steps.  Knowing exactly which steps a student needs and which would be distracting is often not possible.  Direct interaction is a better situation sometimes than sending and reading text & symbols messages.
One suggestion for the step at the "=." line is to use the additive inverse concept, like for some number c,  we can be assured of c+(-c)=0.  The number, c, may stand for any real number, like r, or uw, or xwp, or ab...