who were the first europeans to set up forts along the cost of tanzania and trade gold, ivory, and slaves

The correct answer is - the Portuguese.

The first Europeans to invade the coastline of Tanzania and set up forts on it were the Portuguese. The Portuguese were one of the major colonial powers, and one of the most renowned explorers and traders, so once they reached Tanzania, and saw the potential of the place, they took it over by using military force. The Portuguese were in charge of this part of Africa for most of the 16th century.