an absolute max and min problem involving e

**Find the absolute max and mins of **$\displaystyle f(x)$** on the given interval.**

$\displaystyle f(x)= xe^\frac{-x^2}{8}, [-1,4]$

$\displaystyle f'(x)= e^\frac{-x^2}{8}+\frac{-x^2}{4}e^\frac{-x^2}{8}$

$\displaystyle e^\frac{-x^2}{8}(1-\frac{x^2}{4})^4$

$\displaystyle e^\frac{-x^2}{8}=0$

$\displaystyle e^\frac{-x^2}{8}=0$

=?

$\displaystyle (1-\frac{x^2}{4})^4=0$

$\displaystyle (1-\frac{x^2}{4})=0$

$\displaystyle (\frac{x^2}{4})=1$

$\displaystyle (x^2)=4

$

2,-2

Help?