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