What is a human made structure that is sunk in a region that was previously inhabited by coral reefs?

The answer is artificial reef. Its a man made underwater structure typically built to promote marine life areas with a generally features bottom to control erosion, block ship passage or improve surfing. Artificial reef improve marine life in a certain area and deprive a new location for fishermen to fish and which works to ease the pressure on natural reefs.

2. x2 + 8x + 12 = 0 to prove: x = -6 or x = -2