ay - xy - ab + bx - a + x = y(a - x) + (-ab) + bx + x - a = -y(x - a) + b(x - a) + 1(x - a) = (x - a) (b - y + 1)
ay - xy - ab + bx - a + x = y(a - x) + (-ab) + bx + x - a = -y(x - a) + b(x - a) + 1(x - a) = (x - a) (b - y + 1)