$AA ^{ T }=\left[\begin{array}{lll} a & d & g \\ b & e & h \\ c & f & i \end{array}\right]\left[\begin{array}{lll} a & b & c \\d & e & f \\ g & h & i \end{array}\right]$
Now given $a^{2}+d^{2}+g^{2}+b^{2}+e^{2}+h^{2}+c^{2}+f^{2}+i^{2}=6$
$={ }^{9} C_{3} \times 2^{6}$
$=5376$