a+2b+c=9 (1)
-3a+3b+c=6 (2)
b+c=2 (3)
(3)代入(1)得
a+b+2=9,a+b=7 (4)
(3)代入(2)得
-3a+2b+2=6,-3a+2b=4 (5)
(4)*2-(5)
5a=10
a=2,b=5,c=-3
a+2b+c=9 (1)
-3a+3b+c=6 (2)
b+c=2 (3)
(3)代入(1)得
a+b+2=9,a+b=7 (4)
(3)代入(2)得
-3a+2b+2=6,-3a+2b=4 (5)
(4)*2-(5)
5a=10
a=2,b=5,c=-3