Answer
$$E=\left[\begin{array}{rrr}{0} & {0} & {1} \\ {0} & {1} & {0} \\ { 1} & {0} & {0}\end{array}\right]$$
Work Step by Step
The matrix $$E=\left[\begin{array}{rrr}{0} & {0} & {1} \\ {0} & {1} & {0} \\ { 1} & {0} & {0}\end{array}\right]$$
is elementary and satisfies that
$$EA=\left[\begin{array}{rrr}{0} & {0} & {1} \\ {0} & {1} & {0} \\ { 1} & {0} & {0}\end{array}\right]\left[\begin{array}{rrr}{1} & {2} & {-3} \\ {0} & {1} & {2} \\ {-1} & {2} & {0}\end{array}\right]=\left[\begin{array}{rrr}{-1} & {2} & {0} \\ {0} & {1} & {2} \\ {1} & {2} & {-3}\end{array}\right]=B$$