Solution
= Solution
Undo each <elementary row operation>: divide row 3 by 3, add 3 times row 2 to row 4, and repeat the same swap. Hence
$$
\boxed{E_s^{-1}=\begin{pmatrix}1&0&0&0\\0&1&0&0\\0&0&1/3&0\\0&0&0&1\end{pmatrix},\quad E_a^{-1}=\begin{pmatrix}1&0&0&0\\0&1&0&0\\0&0&1&0\\0&3&0&1\end{pmatrix},\quad E_p^{-1}=E_p.}
$$
Multiplying each inverse by its original matrix gives the <identity matrix>.