23. 已知直线l1:3x−6y+1=0l_{1}:3x-6y+1=0l1:3x−6y+1=0,l2:x−my+2=0l_{2}:x-my+2=0l2:x−my+2=0,l3:nx+y+3=0l_{3}:nx+y+3=0l3:nx+y+3=0,若l1//l2l_{1}//l_{2}l1//l2,且l1⊥l3l_{1}\bot l_{3}l1⊥l3,则m−nm-nm−n的值为((( )))