2y+x+z=13 (1)
3x-9y+z=2 (2)
x+y-z=2 (3)
(1)+(3):2x+3y=15 (4)
(2)+(3):4x-8y=4 (5)
(4)*2-(5):14y=26,y=13/7
代入(4):2x=15-3y=15-3*13/7=(105-39)/7=66/7
x=33/7
z=x+y-2=33/7+13/7-2=20/7-2=(20-14)/7=6/7
x=33/7,y=13/7,z=6/7