Exercise 3.3.31

For $A=\left[\begin{array}{cc}1\hfill & 1\hfill \\ 0\hfill & 2\hfill \\ 0\hfill & 3\hfill \end{array}\right]$, the only solution to $\mathit{Ax}=\left[\begin{array}{c}1\hfill \\ 2\hfill \\ 3\hfill \end{array}\right]$ is $x=\left[\begin{array}{c}0\hfill \\ 1\hfill \end{array}\right].B$ cannot exist since 2 equations in 3 unknowns cannot have a unique solution.