cos(arctan U)
I seriously had this down until my teacher stopped using numbers...
Where the 3 comes from is that Plato is giving you a concrete example of the general question $\displaystyle \tan(\theta) = u, ~ \cos(\theta) = ?$ to try and guide you to answering the general question. You are meant to realise that.
Answer Plato's question ($\displaystyle \tan(\theta) = 3, ~ \cos(\theta) = ?$) and then consider how you might use it as a guide to answering $\displaystyle \tan(\theta) = u, ~ \cos(\theta) = ?$
Also, please don't edit posts after getting replies. For obvious reasons it just creates confusion.