Question

How did kush gain wealth and power

Answer 1

Kush exported iron products of iron hoes, knives and spears.
They exported raw materials frankincense, hides, and carnelian.
Carnelian is a stone used for jewelry and for arrowheads. 
They sent caravans of camels loaded with trade goods into Egypt carrying gold, ivory, and ebony.   
Kush imported cotton textiles and goods from India, Arabia, and China. 
Kush was an active trading center for 600 years, and then it began to decline. Kush began north of the first cataract on the Nile River.
It extended beyond the sixth cataract to present day Khartoum.
Called Kush (Nubia, Nuba, or Aethiopia) it became a rich and powerful nation.
Early culture centered around a settlement at Kerma.
Kerma was a trading center established as an Egyptian trading post.
It was staffed with with Egyptian administrators, soldiers, and artisans,
It was also the residence of the Nubian chief and the center of Nubian government.
Around 2000 BC, nomadic cattle herders existed in the Sudan.
From 800 BC - 350 AD, Kush was a center of culture and military might in Africa.  
Kush was also a land of natural wealth. 
They had gold mines, ivory, incense, and iron ore. 
Unlike Egypt, in Kush, they were not dependent upon the flooding of the Nile.
They had good soil to grow crops and long growing seasons. 
They enjoyed rainfall all year long. 
The Golden Age of Kush was 800 BC to 350 AD.

Answer 2

They won back their independence.