The minors of $-4$ and $9$ and the co-factors of $-4$ and $9$ in the determinant $\left|\begin{array}{ccc}-1& -2& 3\\ -4& -5& -6\\ -7& -8& 9\end{array}\right|$ are respectively:

Let the determinant be $D$.Minor of $-4$ ($a_{21}$): Delete the 2nd row and 1st column.
$$M_{21}=\left|\begin{array}{cc}-2& 3\\ -8& 9\end{array}\right|=(-2)(9)-(-8)(3)=-18+24=6$$
Cofactor of $-4$ ($C_{21}$):
$$C_{21}=(-1{)}^{2+1}{M}_{21}=(-1{)}^3(6)=-6$$
Minor of $9$ ($a_{33}$): Delete the 3rd row and 3rd column.
$$M_{33}=\left|\begin{array}{cc}-1& -2\\ -4& -5\end{array}\right|=(-1)(-5)-(-4)(-2)=5-8=-3$$
Cofactor of $9$ ($C_{33}$):
$$C_{33}=(-1{)}^{3+3}{M}_{33}=(-1{)}^6(-3)=-3$$
The results are $M_{21}=6,M_{33}=-3,C_{21}=-6,C_{33}=-3$.