设：x/a+y/b=1,
-3/a+4/b=1,4a-3b=ab ,a+b=12 ,b=12-a,
4a-36+3a=a(12-a),a^2-5a-36=0,
a=9,b=3; a=-4,b=16
得：x/9+y/3=1,或-x/4+y/16=1
即：x+3y-9=0,或4x-y+16=0.