If the matrix
\[
A=\begin{pmatrix}
-8 & 5
2 & 4
\end{pmatrix}
\]
satisfies the equation \(x^2+4x-k=0\), then the value of \(k\) is ____.
Show Hint
For a \(2\times2\) matrix,
\[
\boxed{
x^2-(\text{Trace})x+\det(A)=0
}
\]
Always verify the question statement if the computed answer does not match the given options.