Va'eirah

In 6,12 Moshe makes a Kal Vachomer "If the Bnei Yisrael didn't listen to me, for sure Pharaoh won't listen to me." But 6,9 just said the Jews didn't listen because of their "shortness of breath and hard work". Those reasons don't apply to Pharaoh, so what's the proof that Pharaoh won't listen?