$3\times 3$  matrices are formed using the elements from the set $\{-5,-3,-1,0,1,3,5\}$  then answer the following number of matrices that are skew symmetric matrices or scalar matrices is 

Let A  be a skew symmetric matrix  $A\left[\begin{array}{ccc}0& x& y\\ -& 0& z\\ -& -& 0\end{array}\right]$  for  $x,y,z$ each can be filled in 7 ways corresponding to each of $x,y,z$  other places can be filled in 1 ways.
 $\therefore n\left(A\right)=7^3$
$B:$  Scalar matrix, then  $n\left(B\right)=7$
$n(A\cap B)=1$ (All entries are zeros)
 $n(A\cup B)=n\left(A\right)+n\left(B\right)-n(A\cap B)=7^3+7.1-1=7^3+6$