Find the matrix $M$ of the linear transformation $T:{\mathbb R}^3 \rightarrow {\mathbb R}^2$ given by $$T\left(\left[\begin{array}{c} x_{1}\cr x_{2}\cr x_{3}\cr \end{array}\right]\right) = \left[\begin{array}{c} 3x_{1}-x_{2}+8x_{3}\cr -6x_{1}+\left(-7\right)x_{2}\cr \end{array}\right].$$
$M=$ $\left[\Rule{0pt}{2.4em}{0pt}\right.$$\left]\Rule{0pt}{2.4em}{0pt}\right.$