Argivian Phalanx

• WOTC Official Reminder Textofficial rules
  Affinity — This spell costs less to cast for each [quality] you control.
• WOTC Official Reminder Textofficial rules
  Vigilance — Attacking doesn't cause this creature to tap.

• WWizards of the Coast2022-09-09
  Official Ruling
  Argivian Phalanx's first ability can't reduce the in its cost.
• WWizards of the Coast2022-09-09
  Official Ruling
  To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as that of Argivian Phalanx). The mana value of the spell is determined by only its mana cost, no matter what the total cost to cast that spell was.

Rulings via Scryfall