Which weights would you use on a single thread to create a 6.86n force

6.86 N = (mass) x (9.8 m/s²)

Mass = (6.86 N) / (9.8 m/s²)

Mass = (6.86 kg-m/s²) / (9.8 m/s²)

Mass = (6.86/9.8) kg

Mass = 0.7 kilogram

I would use the 500-gram weight and the 200-gram weight, and hang them both from the end of the thread.  Then the total mass on the end of the thread is 700 grams.  When I take it to Earth, it will weigh 6.86 N .