$\left[ {\begin{array}{cccccccccccccccccccc}2&{ - 6}\\1&{ - 2}\end{array}} \right]$

.:

Let us take A $= \left[ {\begin{array}{cccccccccccccccccccc}2&{ - 6}\\1&{ - 2}\end{array}} \right]$

We know that, A = IA
$\Rightarrow$ $\left[ {\begin{array}{cccccccccccccccccccc}2&{ - 6}\\1&{ - 2}\end{array}} \right] = \left[ {\begin{array}{cccccccccccccccccccc}1&0\\0&1\end{array}} \right]A$

Applying ${R_1} \to {R_1} - {R_2}$
$\left[ {\begin{array}{cccccccccccccccccccc}1&{ - 4}\\1&{ - 2}\end{array}} \right] = \left[ {\begin{array}{cccccccccccccccccccc}1&{ - 1}\\0&1\end{array}} \right]A$

Applying ${R_2} \to {R_2} - {R_1}$
$\left[ {\begin{array}{cccccccccccccccccccc}1&{ - 4}\\0&2\end{array}} \right] = \left[ {\begin{array}{cccccccccccccccccccc}1&{ - 1}\\{ - 1}&2\end{array}} \right]A$

Applying ${R_1} \to {R_1} + 2{R_2}$
$\left[ {\begin{array}{cccccccccccccccccccc}1&0\\0&2\end{array}} \right] = \left[ {\begin{array}{cccccccccccccccccccc}{ - 1}&3\\{ - 1}&2\end{array}} \right]A$

Applying ${R_2} \to \cfrac{1}{2}{R_2}$
$\left[ {\begin{array}{cccccccccccccccccccc}1&0\\0&1\end{array}} \right] = \left[ {\begin{array}{cccccccccccccccccccc}{ - 1}&3\\{\cfrac{{ - 1}}{2}}&1\end{array}} \right]A$

Hence, ${A^{ - 1}} = \left[ {\begin{array}{cccccccccccccccccccc}{ - 1}&3\\{\cfrac{{ - 1}}{2}}&1\end{array}} \right]$