What details on the map support the idea that Ghana, Mali, and Songhai built wealth on trade?

Ghana, Mali, and Songhai were located near Niger river, in the west of Africa and south of the Sahara Desert.

Explanation:

Trade was extremely important for Ghana, Mali and Songhai because this was the only way for development for these countries. These three empires of pre-colonial period in West Africa earned a lot of profits from the trans-Saharan gold trade. There was demand for gold increasing across the world. People who spoke Berber found trade routes across the Sahara Desert to West Africa.

Gold created a lot of wealth for these countries and they could expand their territories and then gain salt and other needed goods from other countries.