dnd 5e – Can a Warlock use Pact Magic slots to cast spells learned from another spellcaster class?

Yes, you can use warlock slots to cast any spell you know. It’s in the PHB in the section on multiclassing. Page 164:

If you have both the Spellcasting class feature and the Pact Magic class feature from the warlock class, you can use the spell slots you gain from the Pact Magic feature to cast spells you know or have prepared from classes with the Spellcasting class feature, and you can use the spell slots you gain from the Spellcasting class feature to cast warlock spells you know.

The effect is determined by the slot’s level, as always in D&D 5e.