已知向量$\stackrel{\rightharpoonup }{a}$与$\stackrel{\rightharpoonup }{b}$的夹角为$60°$，且$\stackrel{\rightharpoonup }{a}=(-2,-6)$,$\left|\stackrel{\rightharpoonup }{b}\right|=\sqrt{10}$,则$\stackrel{\rightharpoonup }{a}·\stackrel{\rightharpoonup }{b}=$.