Write the system of equations with the given coefficient matrix and right-hand side vector.
$$A=\left[\begin{array}{rrrr}1 & -1 & 2 & 3 \\1 & 1 & -2 & 6 \\3 & 1 & 4 & 2 \end{array}\right], \mathbf{b}=\left[\begin{array}{r}1 \\-1 \\2\end{array}\right].$$