Balance the two groups of 3 marbles each. If they balance then it is only necessary to balance the remaining two marbles to find the heavy one. If the two groups of 3 marbles do not balance, take the heavier group. Of the 3 marbles in the heavier group, balance any 2 marbles. If they balance, the remaining marble is the heaviest one. If the 2 marbles do not balance, the heaviest will be 1 of the 2 on the balance