Q(x) is a part of the equation though?
View attachment 37364
Q(x) is just considered apart of the polynomial- meaning that P(-x)=...Q(-x)...
P(x)+P(-x)={(x^2-5)Q(x)+(x+4)}+{(-x)^2-5)Q(x)+(-x+4)}
then we can factorise the (x^2-5)
therefore,(x^2-5)(Q(x)+Q(-x))+(x+4)+(-x+4)
try letting Q(x)=x
consider (Q(x)+Q(-x))
Q(x)+Q(-x)=x-x=0---->this applies for any equation you let Q(x) be
hence, (x^2-5)(0)+8
=8