In order to solve this problem, we need to have more information. For example, if we knew the total number of chairs in the restaurant, we could solve it as follows:

Let n=(total chairs in restaurant) and w=(fraction of seated tables that still have to order their food)

w=6/[n(2/3)]

n(2/3) lets us know how many tables make up 2/3 of the total tables.

6/[n(2/3)] lets us know the fraction of the 2/3 tables that haven't ordered.

let 'x' represent the total number of tables in the restaurant, then '(2/3)x' represents the number of tables that are taken

six of those tables haven't ordered their food yet and those table still have to order their food

so '6' out of '(2/3)x' tables still need to order their food:

6/((2/3)x)

which simplifies to:

9/x

therefore '9/x' represents the fraction of seated tables that still have to order their food