An airplane flies at an elevation of 500 feet above sea level. A whale swims the same distance below sea level, what number represents the elevation of a whale?

500 below because you just do the opposite

arc length = radius * x ( in radians)

6 * pi = 18 * x

divide by 18 on each side

6 pi / 18 = x

pi/3 = x

the angle is pi / 3 radians

or   60 degrees

3/8