giúp c2 đúng sai

$\displaystyle y=\frac{-x^{3}}{3} +3x^{2} -8x+2$
a, $\displaystyle y'=-x^{2} +6x-8$
b, $\displaystyle y'=0\ $
$\displaystyle  \begin{array}{{>{\displaystyle}l}}
-x^{2} +6x-8=0\ \\
x=4,\ x=2
\end{array}$
c, 
$\displaystyle y( 4) =\frac{-4^{3}}{3} +3.4^{2} -8.4+2=-\frac{10}{3}$
d, 
$\displaystyle  \begin{array}{{>{\displaystyle}l}}
f( -1) =\frac{40}{3}\\
f( 2) =\frac{-14}{3}\\
f( 4) =\frac{-10}{3}\\
f( 8) =\frac{-122}{3} =min
\end{array}$