9=6b*1/2
b=3
9=-6b*1/2
b=-3
设解析式为y=kx+b
6k+b=0
k=-b/6
则k=1/2,-1/2
解析式为y=1/2x-3
y=1/2x+3
y=-1/2x+3
y=-1/2-3