Playing an Oath of Conquest/Warlock Multiclass [closed]

As an Oath of Conquest Paladin I gain the Armor of Agathys as a Paladin spell. IF I am multi classed as a warlock,can I use a Warlock spell slot to cast it? Or are known spells and spell slots completely separate? Would I need to learn Armor of Agythys as one of my Warlock known spells to use as Warlock slot?