Chabad.org — English translation + Rashi
Torah Temimah on this passuk (citation pointer only). Core text below is the Chazal derasha as recorded by TT — not TT’s explanatory voice. The elders must be “known” to Moshe — men already recognized as leaders and officers of the people.
English: “Whom you know to be the elders of the people and its officers.”
Prior recognition — the seventy are drawn from those already trusted.
The new institution rests on existing communal standing.