1.) You have 12 toppings. You choose one topping--that leaves you with 11 toppings. You choose another--that leaves 10. 12×11×10 = 1,320. Multiply 1,320 topping choices by 6 cheeses to get 7,920 total combinations.
2.) (Though I'm less sure of these) CDs: 6×5×4×3×2 = 720 Cassettes: 5×4 = 20 DVDs: 8×7×6×5 = 1680