x^2 - 10x = x^2 - 10x + 0 = x^2 - 10x + (25 - 25) = x^2 - 10x + 25 - 25 = (x - 5)^2 - 5^2 = (x - 5 - 5)(x - 5 + 5) = (x - 10)x
x^2 - 10x = x^2 - 10x + 0 = x^2 - 10x + (25 - 25) = x^2 - 10x + 25 - 25 = (x - 5)^2 - 5^2 = (x - 5 - 5)(x - 5 + 5) = (x - 10)x