You have that the distance between Paulo and the dolphin is 100ft, that is, the length of CD is 100ft.
To find the distance between Rex and the dolphin and Ben and the dolphin, you first use the greater triangle ABC, and also you take into account that the summ of the interoir angles of a triangle is 180°.
Then, for angle C you have:
∠C = 180 - 30 - 60 = 90°
Furthermore, for angle ∠BCD you have:
∠BCD = 180 - 90 - 30 = 60
and for angle ∠ACD:
∠ACD = 180 - 60 - 90 = 30
Now, consider that for a 30-60-90 triangle, the hypotenuse is twice the length of the shorter length. Thus, for the distance between the Ben and the dolphin BC you have a triangle with its shorter leg equal to 100ft.
Then, you have for the hypotenuse h:
h = BC = 2(100ft)
h = BCa = 200ft
Next, for the distance between Rex and the dolphin use the right triangle of the figure. Consider the greater leg is √3 time the shorter one.
Then, for greater leg (side CD) related to shorter leg (side AD) you have:
√3CD = AD replace the value of CD and solve for AD