In 1642, Güshi Khan, founder of the Khoshut Khanate, overthrew the prince of Tsang and made the 5th Dalai Lama the highest spiritual and political authority in Tibet, establishing the regime known as Ganden Phodrang. Tsewang Rabtan invaded Tibet in 1717, deposed the pretender to the position of Dalai Lama of Lha-bzang Khan, the last ruler of the Khoshut Khanate, and killed Lha-bzang Khan and his entire family. They also viciously destroyed a small force in the Battle of the Salween River which the Kangxi Emperor of the Qing dynasty had sent to clear traditional trade routes in 1718. In response, an expedition sent by Kangxi Emperor, together with Tibetan forces under Polhané Sönam Topgyé of Tsang and Kangchennas , the governor of Western Tibet, expelled the Dzungars from Tibet in 1720. This began the Qing rule of Tibet, which lasted until the fall of the Qing dynasty in 1912.
Question: How many years after Tsewang Rabtan invaded Tibet did the governor of Western Tibet, expel the Dzungars from Tibet?
Answer: 3