There is a lot of confusion regarding how to calculate the number of spell slots you get when you multiclass. This is my attempt at explaining it.

Spells Known and Prepared:
You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class.

Spell Slots from the “Spellcasting” class feature:
• Add together all your levels in the bard, cleric, druid, sorcerer, and wizard classes;
• If you have two or more levels in the paladin class, add half these levels (rounded down);
• If you have two or more levels in the ranger class, add half these levels (rounded down);
• If you have three or more levels in the fighter (eldritch knight) class, add one third these levels (rounded down);
• If you have three or more levels in the rogue (arcane trickster) class, add one third…