A and B are square matrices of the same order 3,

Given:

$A$ and $B$ are square matrices of order 3

$|A|=2$

$A B=21$

Now, $A B=2 I \Rightarrow|A B|=|2 I \Rightarrow| A|| B|=| 2 I \mid$

$(\because|A B|=|A| B \mid$, if they are square matrices of same order $) \Rightarrow 2|B|=|2 I|$

$(\because|A|=2) \Rightarrow 2|B|=2^{3}|I|$

$(\because$ Order of $I$ is $3 \times 3) \Rightarrow 2|B|=8 \times 1$

Hence, the value of $|B|$ is 4 .