Albert Barnes Commentary Exodus 9:35

"And the heart of Pharaoh was hardened, and he did not let the children of Israel go, as Jehovah had spoken by Moses." — Exodus 9:35 (ASV)

Hardened - Different words in the Hebrew. In Exodus 9:34 the word means “made heavy,” that is, obtuse, incapable of forming a right judgment; in Exodus 9:35 it is stronger, and implies a stubborn resolution.