Determinant of $\left[\begin{array}{cc}1 & 3\\7 & 9\end{array}\right]$

Calculate $\left|\begin{array}{cc}1 & 3\\7 & 9\end{array}\right|$.

The determinant of a 2x2 matrix is $\left|\begin{array}{cc}a & b\\c & d\end{array}\right| = a d - b c$.

$\left|\begin{array}{cc}1 & 3\\7 & 9\end{array}\right| = \left(1\right)\cdot \left(9\right) - \left(3\right)\cdot \left(7\right) = -12$

$\left|\begin{array}{cc}1 & 3\\7 & 9\end{array}\right| = -12$A