We are given that a dolphin moves describing a projectile motion. This can be represented in the following graph of position vs time:
Since the dolphin moves horizontally as he goes through the hoop this means that the hoop is at the maximum height of the motion. The maximum height of a projectile motion is given by:

Where:

Now, we plug in the values:

Solving the operations:

Therefore, the hoop is at 2.2 meters above the water.