若向量 $\stackrel{\rightharpoonup }{a}=(1,1,x)$, $\stackrel{\rightharpoonup }{b}=(1,2,1)$, $\stackrel{\rightharpoonup }{c}=(1,1,1)$,满足条件 $(\stackrel{\rightharpoonup }{c}-\stackrel{\rightharpoonup }{a})·\left(2\stackrel{\rightharpoonup }{b}\right)=-2$,则 $x$=.