Solve $\small{e^{-2x}-10e^{-x} = -21 \;\mbox{for} \;x}$ without using a calculating utility. Write your answers as a comma-separated list, and express them using the natural logarithm, ln. [Hint: Rewrite the equation as a quadratic equation in $\small{u = e^{-x}}$ .]

$\small{x =}$ { }