由题可知：
3x-2y-4>=0 ===>3x-2y>=4 ===>9x-6y>=12,
2x-7y+3>=0 ===>2x-7y>=-3 ===>8x-28y>=-12,
所以17x-34y>=0,
x-2y>=0,
又-x+2y>=0 ===>x-2y=0 ===>x-2y>=z/2,
所以z