(一)5x-4y+3z=15 (1)
6x+2y+9z=13 (2)
7x+6y-6z=6 (3)
(2)乘以2加（1）得17x+21z=41
(2)乘以3减（3）得11x+33z=33
解得：x=2 y=-1 z=1/3
（二）5x-4y-6z=21 (1)
-2x+3y+4z=-15 (2)
3x-7y-5z=15 (3)
(1)乘以3减（2）乘以4 得：7x-2z=3
(2)乘以7减（3）乘以3 得：5x-13z=60
解得：x=-1 y=1 z=-5