Answer
$M_{12}=-12$
and
$A_{12}=12$
Work Step by Step
The minor of an element is defined $a_{ij}$of a determinant is defined as the determinant obtained by deleting its ith row and jth columns and the minor $M$ of an element $a_{ij}$ is abbreviated as $M_{ij}$.
Further, the cofactor of an element $a_{ij}$ is abbreviated as $A_{ij}$.
where $A_{ij}=(-1)^{i+j} M$
Here, $M_{12}=\begin{vmatrix}-3&2\\0&4\end{vmatrix}=(-3)(4)-0=-12$
and
$A_{12}=(-1)^{1+2} M_{12}=(-1)^3(-12)=12$