?$解:\left\{\begin{array}{l}3 x+2 y=2 k(1) \\5 x+4 y=k+3(2)\end{array}\right.$?
?$(2) -2 (1)得5 x+4 y-6 x-4 y=k+3-4 k$?
?$解得\ x=3 k-3\ $?
?$將\ x=3 k-3\ 代入 (1)得$?
?$9 k-9+2 y=2 k$?
?$解得\ y=\frac{9-7 k}{2}\ $?
?$x+y=3 k-3+\frac{9-7 k}{2}\ $?
?$\because x+y=-5\ $?
?$\therefore 3 k-3+\frac{9-7 k}{2}=-5$?
?$解得\ k=13\ $?
?$ $?
