derivative of ( xe^-x)^1/2
use the chain rule: $\displaystyle \frac d{dx}f(g(x)) = f'(g(x)) \cdot g'(x)$ (here $\displaystyle f(x) = x^{1/2}$ and $\displaystyle g(x) = xe^{-x}$)
or, you can distribute the power so you can use the product rule
note, $\displaystyle \left( x e^{-x}\right)^{1/2} = x^{1/2}e^{-x/2}$
can you continue?