Left behind is a combination of two seats.
So, there are 4 combinations of two seats : {7,9}, {6,9}, {7,8}, and {6,8}.
if we used prolog to get 4 combinations we can write like this in our notepad:
r(6).
r(7).
s(8).
s(9).
set(x):-r(x).
set(y):-s(y).
combination(x,y):-r(x),s(y).
then we can save the notepad with .pl format.
Then its prolog is showed by this picture ;