Suppose that $(m, r, s)$ is a message signed with an Elgamal signature scheme. Choose $h$ with $(h, p − 1) = 1$, and let $r_1 ≡ r^h\pmod p$, $s_1 ≡ sr_1h^{−1}r^{−1}\pmod {p − 1}$, and $m_1 ≡ mr_1r^{−1}\pmod {p − 1}$. Show that $(m_1, r_1, s_1)$ is a valid (forged) signature. Why should this not cause problems?
I don't understand how I am supposed to show this. Any help is appreciated