Đáp án:
$\begin{array}{l}
2f\left( {{x^3}} \right) + f\left( { - {x^3}} \right) = 2x\\
+ Khi:x = 3\\
\Rightarrow 2f\left( {{3^3}} \right) + f\left( { - {3^3}} \right) = 2.3\\
\Rightarrow 2f\left( {27} \right) + f\left( { - 27} \right) = 6\\
+ Khi:x = - 3\\
\Rightarrow 2.f\left( {{{\left( { - 3} \right)}^3}} \right) + f\left( {{3^3}} \right) = 2.\left( { - 3} \right)\\
\Rightarrow 2.f\left( { - 27} \right) + f\left( {27} \right) = - 6\\
\Rightarrow \left\{ \begin{array}{l}
2f\left( {27} \right) + f\left( { - 27} \right) = 6\\
f\left( {27} \right) + 2f\left( { - 27} \right) = - 6
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
f\left( {27} \right) = 6\\
f\left( { - 27} \right) = - 6
\end{array} \right.\\
Vay\,f\left( { - 27} \right) = - 6
\end{array}$
