x+y=5k (1)
x-y=9k (2)
(1)+(2)=> 2x=14k => x=7k
(1)-(2)=> 2y=-4k => y=-2k
代入
2x+2y=6 => x+y=3
=> 7k-2k=3 => 5k=3 => k =3/5