带入P点
2=3k+b
b=2-3k
y=kx+2-3k
令x=0
y=2-3k
令y=0
x=(3k-2)/k
2-3k+(3k-2)/k=12
2k-3k^2+3k-2-12k=0
3k^2+7k+2=0
k=-1/3,k=-2