$ \Rightarrow \left[ {\begin{array}{*{20}{c}}
1&{1 + 2 + 3 + ... + n - 1}\\
0&1
\end{array}} \right] = \left[ {\begin{array}{*{20}{c}}
1&{78}\\
0&1
\end{array}} \right]$
$ \Rightarrow \frac{{n\left( {n - 1} \right)}}{2} = 78 \Rightarrow n = 13, - 12$ (reject)
$\therefore $ we have to find inverse of $\left[ {\begin{array}{*{20}{c}}
1&{ - 13}\\
0&1
\end{array}} \right]$
$\therefore \left[ {\begin{array}{*{20}{c}}
1&{ - 13}\\
0&1
\end{array}} \right]$