Chapter 4.2, Problem 22E

In Problems $19\text{-}22$, write the system of linear equations that is represented by the given augmented matrix. Assume that the variables are $x_1$ and $x_2$.

$\left.\left[\left.\begin{array}{cc}1& -2\\ 0& 1\end{array}\right|\right.\begin{array}{c}12\\ 6\end{array}\right]$