If you have the expression you can factor out -1 by dividing both terms by -1, the terms being and . , and , so you end up with , or . Make sense? Basically you can pull out whatever number you want, as long as you divide accordingly. I could factor 543 out of , and I would end up with --multiply that out if you don't believe that it's the same thing as . In this case they're pulling out a -1.