Answer
$\int3xe^{-x}dx=-3xe^x-3e^{-x}dx+C$
Work Step by Step
We integrate as follows:
$u=3x$
$D(u)=3$
$v=e^{-x}$
$I(v)=-e^{-x}$
$\int u·v~dx=u·I(v)-\int D(u)I(v)dx$
$\int3xe^{-x}dx=-3xe^x-\int 3(-e^{-x})dx=-3xe^x+\int 3e^{-x}dx=-3xe^x-3e^{-x}dx+C$
