It depends. Were you given those potentials, or did you look them up in a table? If they were given, then just plug it into the equation, like @ShadowVegan said. But if you looked them up, one of them should be flipped, and it has nothing to do with the "more positive" one.
Usually, a table gives you only reduction potentials, or only oxidation potentials, but not both. So unless you have two oxidizing agents or two reducing agents, which is impossible, one of them has to be flipped. So if you have a table of standard reduction potentials, for example, the oxidizing agent is the number that remains unchanged, while the reducing agent gets flipped.