a rope is 120 ft long and the water is 20 ft deep. how far is the anchor from a point directly below the boat?

Use Pythagoras theorem,

L = √(H²+D²)
or
L² = H²+D²

L=Length of the rope = 120'
D=Depth = 20'
H=Horizontal distance of the anchor from the boat

From
L² = H²+D²
we get
H² = L² - D²
= 120²-20²
=14400 - 400
= 14000
Horizontal distance
= √H
= 118.3'