A seagull flying 21 feet above sea level is expressed as t (plus) 21 feet. A fish swims the same number of feet below sea level. If sea level is considered to be 0, express the distance that the fish is swimming.

h = - 21 ft

Step-by-step explanation:

Think of this as a vertical number line.

If sea level is zero and the seagull is flying 21 ft above the surface, the bird's height

h = + 21 ft

If the fish is 21 ft below the surface, it is swimming at a level

h = - 21 ft.