5.5 The Substitution Rule/1

${\begin{aligned}&\int {e^{-x}}dx,\quad u=-x\\[2ex]&u=-x\\[2ex]&du=-dx\\[2ex]\end{aligned}}$

$\int {e^{-x}}dx=-\int {e^{u}}du=-e^{u}+C=-e^{-x}+C$