The elementary row operation required is to swap rows (1) and (2) of the first matrix. The reverse operation is simply to swap rows (1) and (2) of the second matrix.
Work Step by Step
Recall from the bottom of page 6 and the top of page 7 that all elementary row operations are reversible. Moreover, there are only three such operations: (1) swapping two rows, (2) multiplying a row by a nonzero constant, and (3) adding a constant multiple of one row to another row.