To find the penguin's position relative to sea level, we start with its initial position, which is 21 feet below sea level. We can represent below sea level as negative:
-21 feet (initial position)
The penguin then swims up 9 feet. To find the new position, we add 9 feet to the initial position:
\[ -21 + 9 = -12 \]
The penguin is now at -12 feet, which means it is 12 feet below sea level.