If the matrix
\[
M=
\begin{bmatrix}
x+5 & a & -4 \\
-2 & 0 & b \\
c & 6 & y+1
\end{bmatrix}
\]
is a skew-symmetric matrix, then the value of
\[
ab+c^2-xy
\]
is:
Show Hint
In skew-symmetric matrices, first make all diagonal entries zero. It immediately gives equations for unknown variables.