I know probably find in the book but kind of confusing to me. I was just wanting to know how many spells should I have being a arcane trickster rogue 7/ Wizard level 2?
You're correct, you can find this material in the books. From the "Multiclassing" section:
You determine your available spell slots by adding together all your levels in the bard, cleric, druid, sorcerer, and wizard classes, half your levels (rounded down) in the paladin and ranger classes, and a third of your fighter or rogue levels (rounded down) if you have the Eldritch Knight or the Arcane Trickster archetypes. Use this total to determine your spell slots by consulting the Multiclass Spellcaster table.
So you'll have two levels of Wizard, and your seven levels of Rogue (Arcane Trickster) round down to two more levels - giving you four 1st level spell slots, and three 2nd level spell slots.