If A and B are square matrices of order 3 such that |A| = − 1, |B| = 3,

$|K A|=K^{n}|A|$$[n$ is the order of $A]$

$\Rightarrow|3 A B|=3^{3}|A B| \quad \ldots(1)$

If $A$ and $B$ are square matrices of the same order, then $|A B|=|A||B| .$ So,

$|3 A B|=3^{3}|A||B| \quad$ [From eq. (1)]

$=27 \times-1 \times 3$

$=-81$