Algebra of Function

By assigning $x$ as $2$ , we get

$f(2)+2f(-1)=2$

while assigning $x$ as $-1$ , we would have

$f(-1)-f(2)=\frac{1}{2}$

$\leftrightarrow 2f(-1)-2f(2)=1$ .

By subtracting first to second equation, we get $f(2)=\frac{1}{3}$ which would reveal the $a$ , $b$ , and the solution, $13$ .

Put x=2 and x=-1 in given equation then we have two values of f(-1); 1.f(-1)=1-(a/2b) ; x=2 and 2.f(-1)=0.5+(a/b); x=-1. Then after solving this we have a:b=1:3=f(2). So, for this value ans is 13