21.实数xxx,yyy满足−1<x−y<2-1<x-y<2−1<x−y<2,1<3x+y<41<3x+y<41<3x+y<4,那么x−2yx-2yx−2y的取值范围是 ___(−114,134)(-\frac{11}{4},\frac{13}{4})(−411,413)___.