a-2b-c=0(1)
2a+b+c=0(2)
(1)+(2)得:3a-b=0,3a=b.a:b=1:3
(2)*2得:4a+2b+2c=0(3)
(1)+(3)得:5a+c==,5a=-c,a:c=1:(-5)
所以a:b:c=1:3:(-5)