Answer
$5$
Work Step by Step
We know that for a matrix
\[
\left[\begin{array}{rr}
a & b \\
c &d \\
\end{array} \right]
\]
the determinant $D$ is given by the formula $D=ad-bc.$
Hence, the given matrix has
$D=(3\cdot3)-(4\cdot1)\\
D=9-4\\
D=5$
