已知向量 $\stackrel{\rightharpoonup }{a}=(-2,-1)$， $\stackrel{\rightharpoonup }{a}\bullet \stackrel{\rightharpoonup }{b}=10$, $\stackrel{\rightharpoonup }{|a}-\stackrel{\rightharpoonup }{b|}=\sqrt{5}$，则 $\left|\stackrel{\rightharpoonup }{b}\right|=$       ．