I'm confused about this problem:

$\displaystyle f(x)=e^{\frac{-x^2}{2}}$, find $\displaystyle f'(x)$

My textbook says the answer is:

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

However, there isn't an explanation of the first step. Where does $\displaystyle (-\frac{2x}{2})$ come from? Could someone explain this to me?

Thanks,