已知向量 $\stackrel{\rightharpoonup }{a}=\left(\sqrt{3},1\right),\stackrel{\rightharpoonup }{b}=\left(0,-1\right),\stackrel{\rightharpoonup }{c}=\left(k,\sqrt{3}\right)$。若 $\stackrel{\rightharpoonup }{a}-2\stackrel{\rightharpoonup }{b}$与 $\stackrel{\rightharpoonup }{c}$共线，则 $k$=.