rule
f(x^2+x+3)+2f(x^2-3x+5)
=k(x？ +x+3)+b+2k(x？ -3x+5)+2b
=3kx？ -5kx+ 13k+3b
therefore
3k=6
-5k=- 10
13k+2b= 17
The solution is k=2 and b=-4.5.
f(20 15)=2*20 15-4.5
=4025.5