\[\begin{array}{l}
f\left( x \right) = \left\{ \begin{array}{l}
{x^2} + 3x\,\,\,khi\,\,\,x > 0\\
1 - x\,\,khi\,\,\,x < 0
\end{array} \right.\\
\Rightarrow S = f\left( 1 \right) + f\left( { - 1} \right) = \left( {{1^2} + 3.1} \right) + \left( {1 - - 1} \right)\\
= 1 + 3 + 2 = 6.
\end{array}\]