$\stackrel{\rightharpoonup }{a},\stackrel{\rightharpoonup }{b}$的夹角为 ${120}^{°}$， $\left|\stackrel{\rightharpoonup }{a}\right|=1,\left|\stackrel{\rightharpoonup }{b}\right|=3$，则 $\left|5\stackrel{\rightharpoonup }{a}-\stackrel{\rightharpoonup }{b}\right|$=。