11x - xy + 11y - x^2 = x(11-y) + 11(y-x)

11x - xy + 11y - x^2 = -x(x-11) - y(x-11)

11x - xy + 11y - x^2 = (x-11)(-y) + (x-11)(11)

11x - xy + 11y - x^2 = (x-11)(11-y)