Hi, I'm after some help please.

heres the question I've been given:

An observer sees a ship from the top of a cliff of height 100m. The ship is seen through an instrument which measured the angle of depression to be 80o. The observer then moved back a distance of 10m, at which point the ship and the top of the cliff were in line. Find the distance from the base of the cliff to the ship.

thanks in advance