Matrices And Determinants

If the matrix $M=\left[\begin{array}{ccc}x+5 & a & -4 \\ -2 & 0 & b \\ c & 6 & y+1\end{array}\right]$ is a skew symmetric matrix, the value of the expression $\boldsymbol{a} \boldsymbol{b}+\boldsymbol{c}^{\mathbf{2}}-\boldsymbol{x} \boldsymbol{y}$ is: