cubercalebbtaylor2401Since subtraction is anticommutative I don't think that you can make that jump. But I am not entirely sure.

= sum((x*x - 2*x*u + u*u)) / n

= (sum(x*x) - sum(2*x*u) + sum(u*u)) / n

-2xu = -1*2xu

It's just pulling out a constant multiplier, which comes out in front of a summation. (Because ab+ac = a(b+c)). This is the same rule that lets me pull out the 2u on line 4.