136. Find the value of $f(2)$, if for any $x \neq 0$ the equality holds

$$
f(x)+3 f\left(\frac{1}{x}\right)=x^{2}
$$
Solution. Substituting $x=2$ and $x=\frac{1}{2}$ into the given equation, we will have:

$$
f(2)+3 f\left(\frac{1}{2}\right)=4, f\left(\frac{1}{2}\right)+3 f(2)=\frac{1}{4}
$$

Now, subtracting the second equation, multiplied by 3, from the first equation, we get $f(2)=-\frac{13}{32}$.

Answer: $f(2)=-\frac{13}{32}$.