$$\eqalign{
& y = 3f\left( {x + 2} \right) - {x^3} + 3x \cr
& y' = 3f'\left( {x + 2} \right) - 3{x^2} + 3 \cr
& Dat\,\,t = x + 2 \cr
& \Rightarrow x = t - 2 \cr
& \Rightarrow y' = 3f'\left( t \right) - 3{\left( {t - 2} \right)^2} + 3 \cr
& y' = 3f'\left( t \right) - 3{t^2} + 12t - 9 \cr
& = 3f'\left( t \right) - 3\left( {{t^2} - 4t + 3} \right) \cr
& = 3f'\left( t \right) - 3\left( {t - 1} \right)\left( {t - 3} \right) \cr
& + )\,\,t \in \left( { - \infty ;1} \right) \cr
& \Rightarrow \left\{ \matrix{
f'\left( t \right) < 0 \hfill \cr
\left( {t - 1} \right)\left( {t + 3} \right) > 0 \Rightarrow - 3\left( {t - 1} \right)\left( {t + 3} \right) < 0 \hfill \cr} \right. \cr
& \Rightarrow 3f'\left( t \right) - 3\left( {t - 1} \right)\left( {t - 3} \right) < 0 \cr
& \Rightarrow Ham\,\,so\,\,NB/\left( { - \infty ;1} \right) \cr
& + )\,\,t \in \left( {1;3} \right) \Rightarrow \left\{ \matrix{
f'\left( t \right) \ge 0 \hfill \cr
\left( {t - 1} \right)\left( {t + 3} \right) < 0 \Rightarrow - 3\left( {t - 1} \right)\left( {t + 3} \right) > 0 \hfill \cr} \right. \cr
& \Rightarrow y' > 0 \Rightarrow Ham\,\,so\,\,DB/\left( {1;3} \right) \cr
& + )\,\,t \in \left( {3;4} \right) \cr
& \Rightarrow \left\{ \matrix{
f'\left( t \right) < 0 \hfill \cr
\left( {t - 1} \right)\left( {t + 3} \right) > 0 \Rightarrow - 3\left( {t - 1} \right)\left( {t + 3} \right) > 0 \hfill \cr} \right. \cr
& \Rightarrow y'\,\,ko\,\,xac\,\,dinh\,\,dau \Rightarrow Ko\,\,KL\,\,ham\,\,so\,\,DB\,\,hay\,\,NB \cr
& + )\,\,t \in \left( {4; + \infty } \right) \cr
& \Rightarrow \left\{ \matrix{
f'\left( t \right) > 0 \hfill \cr
\left( {t - 1} \right)\left( {t + 3} \right) > 0 \Rightarrow - 3\left( {t - 1} \right)\left( {t + 3} \right) < 0 \hfill \cr} \right. \cr
& \Rightarrow y'\,\,ko\,\,xac\,\,dinh\,\,dau \Rightarrow Ko\,\,KL\,\,ham\,\,so\,\,DB\,\,hay\,\,NB. \cr
& Vay\,\,ham\,\,so\,\,DB/\left( {1;3} \right) \cr} $$
