Answer: She sell 2.6 kg of pears at the farmer's market.
Explanation:
Given: The weight of apples and pears together = 6.5 kg
Weight of apples =
of (total apples and pears)
Weight of apples =

Weight of pears = 6.5-3.9 kg = 2.6 kg
Hence, She sell 2.6 kg of pears at the farmer's market.