4x-9z=17 (1)
3x+y+15z=18 (2)
x+2y+3z=2(3)
(2)*2-(3)
5x+27z=34 (4)
(1)*3+(4)
17x=85
x=5
z=(4x-17)/9=1/3
y=18-15z-3x=-2