What important natural resource in west africa did the songhai empire control?

The important natural resource in West Africa that the Songhai Empire controlled was salt.
This salt was predominantly found in the Sahara desert - there are still a lot of salt mines in this West African desert. The Songhai Empire heavily relied on salt trade, which brought most of its money into the empire - this is the reason why it was so powerful at the time - because of salt and gold. 

They made a checks and balance system to equal power within the branches. the "check" meant that one branch would watch one branch and "balance" them if they were too powerful. if you want more info on this, i would just google checks and balance system.