What you mean is (A^T)-1 and your trying to find the orginal matrix A. Well first find the determinant of [1 2 3] [2 0 1] [1 1 -1 ] which is 11. Multiply everything by 11. To get A^T which is [11 22 33][22 0 11][11 11 -11]. Then undo the transpose which would be [11 22 11][22 0 11][33 11 -11] and reduce the matrix by 1/11 which = [1 2 1][2 0 1][3 1 -1].