which west african empire worked with iron and prospered from gold-salt trade

The West African empire that worked with iron and prospered from the gold-salt trade was the Mali Empire.

which west african empire created a code of laws, made timbuktu the center of Islam scholarship, expanded it's empire to 50 million people, and prospered from trade

The West African empire that fits this description is the Songhai Empire.

which west african empire maintained a strong military, established a government system, and became the largest of the west african empires

The West African empire that maintained a strong military, established a government system, and became the largest of the West African empires was the Ghana Empire.