2X+y=10.(1)
4y+z=21.(2)
5X-z=10.(3)
(2)+(3)得：4y + 5X =31.(4)
(1)×4 得：8X + 4y =40.(5)
(5)-(4)得：3X =9
X=3
将X=3代入(1)中,得y=4
将X=3代入(3)中,得z=5