若 $x$， $y$满足约束条件 $$\begin{gathered} \left\{\begin{array}{l}x-3y\le 1 \\ x+2y\le 9 \\ 3x+y\ge 7\\ \end{array}\right. \end{gathered}$$，则 $z=2x-y$的最大值为＿＿＿＿＿＿．