Let the maximum and minimum values of \( \, (\sqrt{8x - x^{2} - 12} - 4)^{2} + (x - 7)^{2} \, \), \( x \in \mathbb{R} \) be \( M \) and \( m \), respectively. Then \( M^{2} - m^{2} \) is equal to \( \_\_\_\_\_\_ \).