The determinant of A is the sum of the entries in ANY row or column of A multiplied by their respective cofactors. In this case, let’s choose row 1 because there is a zero entry in the second column of the first row. (We could also choose the second column, third column, and third row since they all have a 0 entry).

The determinant of

Since a₁₂=0 à |A|=a₁₁C₁₁+a₁₃C₁₃

C₁₁=(-1)¹⁺¹M₁₁=M₁₁=8

C₁₃=(-1)¹⁺³M₁₃=M₁₃=6

|A|=a₁₁C₁₁+a₁₃C₁₃=3(8)+1(6)=30