I have a matrix:

$\displaystyle \begin{bmatrix}x-4&4&0\\-1&x&0\\0&0&x-5 \end{bmatrix}$

I believe I've correctly used cofactor expansion to get:

(x-5)^(3+3) det($\displaystyle \begin{bmatrix}x-4&4\\-1&x \end{bmatrix})$

I'm not sure what to do from this point, or if I'm doing it correctly at all... Help appreciated.

For instance, how would I find the cofactor of the new matrix?