6.已知a→=(1,−2,y)\overrightarrow{a}=(1,-2,y)a=(1,−2,y),b→=(x,2,1)\overrightarrow{b}=(x,2,1)b=(x,2,1),2b→//(a→−b→)2\overrightarrow{b}//(\overrightarrow{a}-\overrightarrow{b})2b//(a−b),则((( )))