What is the test for establishing that appellant used both a knife and a firearm in the commission of the crimes?

Appellant fails to acknowledge the testimony of the victims establishes that appellant used both a knife and a firearm in the commission of the crimes. "[U]nless the testimony is physically impossible or inherently improbable, testimony of a single witness is sufficient to support a conviction.' " (People v. Mejia (2007) 155 Cal.App.4th 86, 93.) Our role is not to determine credibility of witnesses. " '[I]t is the exclusive province of the trial judge or jury to determine the credibility of a witness and the truth or falsity of the facts on which that determination depends.' " (People v. Hovarter (2008) 44 Cal.4th 983, 996.)