Mercy and truth preserve the king
Which are two good qualifications in a prince; not ruling his subjects with rigour and cruelty, but with tenderness and clemency; easing them as much as he can of burdens and pressures; showing compassion to the distressed, and pardoning delinquents when the case will admit of it; as also being faithful to his word, promises, and engagements; inviolably adhering to the laws and constitution of the nation, and steady in his administrations of justice; these preserve him in the affections of his people, and make him safe and secure on his throne; and because of these the Lord preserves him from his enemies. It maybe rendered, "grace and truth" F12; and applied to Christ, who is full of both, and which are said to preserve him, ( Psalms 40:11 ) ; and his throne is upholden by mercy;
this explains what is meant by the preservation of him, and what is the security of his throne and kingdom, which is clemency and goodness to his subjects.
The Brenton translation of the Septuagint is in the public domain.