I am sure. Literally, "I know," a better rendering, since, "I am sure" implies something leas than knowledge. No, not by a mighty hand. Or "not even by a mighty hand." Pharaoh will not be willing to let you go even when my mighty hand is laid upon him.
(See Exodus 8:15, Exodus 8:19, Exodus 8:32; Exodus 9:12, Exodus 9:35; Exodus 10:20, Exodus 10:27.) "But by strong hand" (marg.) is a rendering which the rules of grammar do not permit.