Should be 2. Divide into groups of 3/3/2. Weigh 3 against 3. If equal, take the 2 and weigh one against the other to find heavier ball (2 weighings total). If not equal, pick the heavier group of 3 - weigh 1 against 1 and leave the other ball to one side. If equal, heavy ball is 1 not on scales. If not equal, you have found the heavier ball (2 weighings total)