y=-7x+22
先求出斜率为-7
求法:
(k-3)/(1+3k)=(3-1)/(1-3*1)=>k=-7
交点是(-5/2,-9/2)
x-y-2=0
｛ ＝＞交点坐标
3x-y+3=0
设y=-7x+b,代入交点解出b=22