Answer
$$
\left[ \begin{array}{ccc} 1&7&-3\\ 0&-4&4\\0&0&-7\end{array} \right]=28.
$$
Work Step by Step
Given
$$
\left[ \begin{array}{ccc} 1&7&-3\\ 1&3&1\\4&8&1\end{array} \right].
$$
Now, adding $-1$ times the first row to the second row, adding $-4$ times the first row to the third row, we get
$$
\left[ \begin{array}{ccc} 1&7&-3\\ 0&-4&4\\0&-20&13\end{array} \right].
$$
Adding $-5$ times the second row to the third row, we get
$$
\left[ \begin{array}{ccc} 1&7&-3\\ 0&-4&4\\0&0&-7\end{array} \right].
$$
Hence, one can see easily that
$$
\left[ \begin{array}{ccc} 1&7&-3\\ 0&-4&4\\0&0&-7\end{array} \right]=28.
$$
