A triangle has vertices $$P_{}^{}=(-8,5)$$, $$Q_{}^{}=(-15,-19)$$, and $$R_{}^{}=(1,-7)$$. The equation of the bisector of $$\angle P$$ can be written in the form $$ax+2y+c=0_{}^{}$$. Find $$a+c_{}^{}$$.

(第八届AIME1990 第7题)