$A = [a_{ij}]$ is a skew-symmetric matrix of order 3 with $a_{12} = 4$. What is $a_{21}$?

Explanation

1. 01

   A skew-symmetric matrix has $A' = -A$, that is $a_{ji} = -a_{ij}$.

2. 02

   So $a_{21} = -a_{12} = -4$.