M = beginbmatrix3&4&6\2&1&−1\−1&3&5endbmatrix
|M| = 3beginbmatrix1&−1\3&5endbmatrix - 4beginbmatrix2&−1\−1&5endbmatrix + 6beginbmatrix2&1\−1&3endbmatrix
3(5 + 3) - 4(10 - 1) + 6(6 + 1) = 24 - 36 + 42 = 30
