I figured this one out. First divide into 3 and 3 with 2 extra marbles on the side.
xxx xxx xx
Weigh the 2 sets of 3
xxx ---- xxx
If they weigh the same, just weight the extra 2 to find the heavy marble.
If one is heavier, remove the lighter set and select 2 from the heavier set.
x ---x x
If they weigh the same, the heavy marble was the outside one. If one is heavier you also know the heavy marble.