题目已知a:b:c=1:3:6a:b:c=1:3:6a:b:c=1:3:6,a+b+c=30a+b+c=30a+b+c=30,那么c−b−a=______.c-b-a=\_\_\_\_\_\_.c−b−a=______.知识点：代数式求值、解三元一次方程组章节：未标注
