Let $\chi_M$ be the characteristic polynomial of $M$. We write it as $\chi_M = X^r Q$ where $r$ is an integer and $Q$ is a polynomial whose constant coefficient is nonzero. Briefly justify that
$$\mathbb{C}^{m+n} = \ker M^r \oplus \ker Q(M)$$
and verify that these subspaces are stable under $H$.