You're given that \(d = k\sqrt{h}\) for some constant \(k\). You can find said constant by substituting \(d=4.8, \ h = 1.8\). Note also that height is in metres while distance is in kilometres - so you to make it easier, you can make the necessary conversion. You don't have to, but it just means you have to remember your units really well. Also, it is implied that you should assume the person in the question is standing up at sea level.
Thus, we have that \(k = \frac{4800}{\sqrt{1.8}} \implies \ d = \frac{4800\sqrt{h}}{\sqrt{1.8}}\).
The first question basically tells you that at the top of the tower, they can see \(\frac{4800\sqrt{5.8}}{\sqrt{1.8}} m\) into the distance. This is implied to be the same distance from the person to the top of the mast. A diagram drawn with this information should look something like the one below.
From here, you can use Pythagoras' Theorem to find the distance from the yacht to the tower (which is exactly on the coast).
Totally understand your confusion as there are a lot of assumptions you have to make - not the best question in the world imo
Hope this helps