5.5 The Substitution Rule/1

Failed to parse (unknown function "\begin{align}"): {\displaystyle \begin{align} & \int{e^-x} dx \\[2ex], u=-x & \int{e^-x} dx \\[2ex] & \-int{e^u} du \\[2ex] & -\e^u du\\[2ex] &= -\e^-x+C \end{align} }