You cast the spell at the level of the slot you use. If you use a warlock slot it is at the warlock slot lever, if you use a wizard/sorcerer slot it is at the level of that slot.
Pact Magic
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.