Exercise 2.1.23

$A=\left[\begin{array}{ccccc}1\hfill & 2\hfill & \text{;}\hfill & 3\hfill & 4\hfill \end{array}\right]$and $x={\left[\begin{array}{cc}5\hfill & -2\hfill \end{array}\right]}^{\prime }$or $\left[\begin{array}{ccc}5\hfill & \text{;}\hfill & -2\hfill \end{array}\right]$and $b={\left[\begin{array}{cc}1\hfill & 7\hfill \end{array}\right]}^{\prime }$or $\left[\begin{array}{ccc}1\hfill & \text{;}\hfill & 7\hfill \end{array}\right]$ $r=b-A\ast x$ prints as two zeros.