Current calculations
You need to calculate using energy. For this problem Wh (watt-hour) is the most convenient.

Batteries are normally rated in A-h (Amp-Hour), not A/h (amps/hour) as you have specified 

Output Energy = 30W * 2 hours = 60 Wh

Input energy = 3.5 A-h * 12 V = 42 Wh

So, you need more than a 3.5 A-h battery. And this doesn't even account for inefficiencies or the fact that many batteries shouldn't be discharged all the way if you want a long life.