6x+15y+12z=18 (1)
6x+2y-14z=-8 (2)
(1)-(2)得13y+26z=26,即y+2z=2 (3)
2x+5y+4z=6 (4)
15x+5y-35z=-20 (5)
(4)-(5)得-13x+39z=26,即3z-x=2 (4)
(3)*2得2y+4z=4 (6)
(6)-(4)得2y+z+x=2,即原题等于4