5.5 The Substitution Rule/1

Failed to parse (Conversion error. Server ("https://en.wikipedia.org/api/rest_") reported: "Cannot get mml. Server problem."): {\displaystyle {\begin{aligned}&\int {e^{-x}}dx,u=-x\\[2ex]&u=-x\\[2ex]&du=-1dx\\[2ex]&\int {e^{-x}}dx=\\[2ex]&-\int {e^{u}}du\\[2ex]&-e^{u}du\\[2ex]&-e^{-x}+C\end{aligned}}}