$${x^2} + {y^2} - 6x - 6y + 4 = 0$$, $${x^2} + {y^2} - 2x - 4y + 3 = 0$$, $${x^2} + {y^2} + 2kx + 2y + 1 = 0$$. If the radical centre of the above three circles exists, then which of the following cannot be the value of k?

If the circles $${x^2} + {y^2} - 2x - 2y - 7 = 0$$ and $${x^2} + {y^2} + 4x + 2y + k = 0$$ cut orthogonally, then the length of the common chord of the circles is