12. Let variables $\mathrm { x } , \mathrm { y }$ satisfy the constraints $\left\{ \begin{array} { c } x + y \leq 4 \\ x - y \leq 2 \\ 3 x - y \geq 0 \end{array} \right.$. Then the maximum value of $3 x + y$ is $\_\_\_\_$.
12. Let variables $\mathrm { x } , \mathrm { y }$ satisfy the constraints $\left\{ \begin{array} { c } x + y \leq 4 \\ x - y \leq 2 \\ 3 x - y \geq 0 \end{array} \right.$. Then the maximum value of $3 x + y$ is $\_\_\_\_$.