How long has slavery exsisted

It depends on what you mean because slavery is still alive today unfortunately in other countries. I'm assuming you mean African American slavery in the USA. Your answer would be from the 1600s - 1865. But at the same time, not every state started slavery at the same time. For instance, Georgia didn't start until the 1700s so it varies, but the first state was Massachusetts in 1619.

the first settlements began along the huang river because of the importance of the huang river in the beginning of china. it goes back to the story, “the tale of emperor yu”, which references the tale of the great flood, which was caused by the huang river and was believed to create and influence china’s making.

b) to encourage free trade