Write the matrix of the quadratic form $$Q(\vec{x}) = x_{2}^{2}-6x_{1}^{2}-6x_{3}^{2}+5x_{1}x_{2}+9x_{1}x_{3}+x_{2}x_{3}.$$
$A=$ $\left[\Rule{0pt}{3.6em}{0pt}\right.$$\left]\Rule{0pt}{3.6em}{0pt}\right.$