Question 9623
Here's one way:

Take two of the marbles and put them to one side.

Put the remaining six marbles on the balance, three on each side. (Weighing # 1)

If they balance then you know that the heavier marble is on of the two marbles you put aside. Weigh these two to find the heavier marble.(Weighing # 2)

If the six marbles (three on a side) don't balance, then take the three that are on the heavy side, put one of these aside and weigh the other two. (Weighing # 2).  If these balance, then you know that the one marble you put aside is the heavier one.
If they don't balance, then you have found the heavier marble on the balance.