How do we know Jesus is both king and priest?

Psalm 110 clearly states that Jesus is appointed a priest forever after the order of Melchizedek, establishing His role as High Priest. This is underscored by the fact that He is also declared to be King at the Father's right hand. In the New Testament, various authors repeatedly reference this psalm to affirm Jesus' dual role. For instance, Hebrews 1:3 emphasizes His exaltation and reign, confirming that He holds both offices perfectly and simultaneously, a position no other Old Testament figure achieved.