（1）y=x-k
y=-2x+3k-1
x-k=-2x+3k-1
3x=4k-1
x=(4k-1)/3＞0
k＞1/4
(2）x=y+k
y=-2x+3k-1
y=-2×(y+k)+3k-1
3y=k-1
y=(k-1）/3＜ 0
k＜ 1
所以1/4＜ k＜ 1