Answer
$4$.
Work Step by Step
We know that for determinants:
(i) if two rows/columns were interchanged, the value of the original determinant gets multiplied by $-1$
(ii) if any row/column was multiplied by a constant $k$, the value of the original determinant gets multiplied by $k$
(iii) if any row/column was multiplied by a constant $k$ ($k\ne0$), and then this is added to another row/column, the value of the original determinant remains unchanged
Hence here because $-1$ times column $1$ is added to column $3$, according to Rule (iii) the value of the determinant is $4$.
