Ὦ Πρωταγόρα τε καὶ Σώκρατες: out of respect for Protagoras, Socrates naturally reverses the order of persons in the common address, cf. 311 d, 316 a. This order is found also in 353 a.
αὐτὸ τοῦτο: in itself; justice in the abstract, as distinguished from the concrete objects which possess the quality. “Justice in itself must be just.” Cf. τοῦτο αὐτὸ τὸ πρᾶγμα below, and 360 e, 1. 3.
ἔστιν ἄρα κτἑ.: is a further question of the supposed interlocutor.