The history of a unified Tibet begins with the rule of Songtsan Gampo (604650 CE), who united parts of the Yarlung River Valley and founded the Tibetan Empire. He also brought in many reforms, and Tibetan power spread rapidly, creating a large and powerful empire. It is traditionally considered that his first wife was the Princess of Nepal, Bhrikuti, and that she played a great role in the establishment of Buddhism in Tibet. In 640 he married Princess Wencheng, the niece of the powerful Chinese emperor Taizong of Tang China.
Try to answer this question if possible (otherwise reply "unanswerable"): What empire did Gampo Songstan found?
unanswerable