Solve $f(5)$ when its given the function

Question

It is given the function $f(x)-3f(8-x)=x$

How can I find $f(5)$?

Answer 1

HINT:

Put $x=a,8-a$ and solve the two simultaneous linear equations for $f(a)$

Answer 2

With $f(3)=3+3f(5)$ you get $$f(5)=5+3f(3)=5+3(3+f(5))=5+9+3f(5)$$ $$2f(5)=-14$$ $$f(5)=-7$$