tanα=h/d  where h=height of the plane and d is the distance from plane to a point above the atoll.
d=h/tanα  but this would be in meters and we need kilometers so
d=(h/tanα)(km/1000m)
d=h/(1000tanα), we are given that α=17 and h=485 then:
d=485/(1000tan17)
d≈1.586km  (to the nearest thousandth of a km or to nearest meter)