Given, l+m−n=0 & 3l2+m2+cln=0
Now taking n=l+m and putting in 3l2+m2+cln=0 we get,
3l2+m2+cl(l+m)=0
⇒3l2+m2+cl2+clm=0
⇒(3+c)l2+clm+m2=0
⇒(3+c)(ml)2+c(ml)+1=0 ..equation (i)
∵ lies are parallel.
So, roots of equation (i) must be equal
⇒D=0
⇒c2−4(3+c)=0
⇒c2−4c−12=0
⇒(c−6)(c+2)=0
c=6 or c=−2
So, positive value of c=6