(x1-x2)(x1+x2)/8+(y1-y2)(y1+y2)/6=0
而P是MN的终点,故x1+x2=2,y1+y2=4
故1/4 + k*2/3=0
故k=-3/8
故方程为y=-3(x-1)/8+2
即y=-3x/8+19/8