设向量 $\stackrel{\rightharpoonup }{a}=(1,2),\stackrel{\rightharpoonup }{b}=(2,3)$，若向量 $\lambda \stackrel{\rightharpoonup }{a}+\stackrel{\rightharpoonup }{b}$与向量 $\stackrel{\rightharpoonup }{c}=(-4,-7)$共线，则 $\lambda =$.