Separate them into 3s. Measure 3 vs 3 while the other 3 sit out. If the measured balls are the same weight then the heavier ball must be in the left out group. Measure 1 and 1 of that group, if the weight is the same it must be the one you did not measure. If the initial measurement of the 3 vs 3 is not the same, do the same procedure with the heavier set of balls.