Solve $\displaystyle e^{-2x}-3e^{-x}=-2$ for x without using a calculator

Now the first step i did was make it into a quadratic equation

$\displaystyle e^{-2x}-3e^{-x}+2=0$

Then i let $\displaystyle u=e^{-x}$

which gives me $\displaystyle u^{x}-3u+2=0$

now im kinda stuck on the next step becuase of the $\displaystyle u^{x}$ in the equation, not sure if i started of wrong or if im missing something thats right in front of me