Answer here (block to see clearly)
Weigh six of the balls, three on each side of the balance. If it remains balanced then one of the two remaining balls is the heavier. Put those two on the scales to show which it is.
If one group of three balls was heavier put any two of those on the scales, if they balance the other one of the three is heavier or if they don't then you can see which one tipped the scales.
simples!