If we have n minus k factorial, that is the same thing as n minus k times n minus k minus 1 times n minus k times n minus k minus 2, all the way down to 1. When you divide these, the 1's are going to cancel out. What you may or may not realize, and you can work out the math, is everything is going to cancel out here until you're just left with up here, everything from n times n minus 1 to n minus k minus 1. If you expand this out or if you distribute this negative number, this is the same thing as n minus k plus 1. n minus k plus 1 is the integer that's 1 larger than this right over here. If you were to divide it out, this would cancel with something up here, this would cancel with something up here, this would cancel with something up here. What you're going to be left with is exactly this thing over here. If you don't believe me, we can actually try it out.