Chapter 4.2, Problem 23E

In Exercises 1–28, compute the products. Some of these may be undefined. Exercises marked should be done by using technology. The others should be done in two ways: by hand and by using technology where possible. [HINT: See Example 3.]

$\left[\begin{array}{ccc}1& 0& -1\\ 2& -2& 1\\ 0& 0& 1\end{array}\right]\left[\begin{array}{ccc}1& -1& 4\\ 1& 1& 0\\ 0& 4& 1\end{array}\right]$