>>921
a=r^2-2rp+1 , b=r-p , c=1
d=r^2-2rq+1 , e=r-q , f=1

af+cd-2be=r^2-2rp+1+r^2-2rq+1-2(r-p)(r-q)
=-2pq+2=-6+2=-4