Matrix
Determinant Calculation Determinant of 2x2 matrix
$$ A= \begin{bmatrix} a & b \newline c & d \end{bmatrix} $$
$|A| = det(A) = ad -bc$
Determinant of 3x3 matrix, also called expansion of the determinant by first row. Link.
$$ B= \begin{bmatrix} a & b & c \newline d & e & f \newline g & h & k \end{bmatrix} $$
$|B| = det(B) = a\begin{vmatrix} e & f \newline h & k \end{vmatrix} -b\begin{vmatrix} d & f \newline g & k \end{vmatrix} +c\begin{vmatrix} d & e \newline g & h \end{vmatrix}$……