I haven't been active here for a long time, so I will post an answer :)
\(\begin{array}{rcl} f(x) &=& xe^{-x} + 3x\\ f'(x) &=& x\dfrac{d}{dx} \left(e^{-x}\right)+e^{-x}\dfrac{d}{dx} \left(x\right) + 3\dfrac{d}{dx} x\\ &=& x\left(-e^{-x}\right) + e^{-x} + 3\\ &=& (1 - x)e^{-x} + 3 \end{array}\)