This question has two answers posted here. The first one uses the trigonometric identity for the tangent of a sum: it requires that you have memorized these identities, but I think it is slightly easier and quicker to work through it. The second one just uses Pythagoras’s theorem: it doesn’t require you to know any formulae, but it requires a bit more working to get to the answer.
Version using trigonometric identities
Version using Pythagoras
Thanks to Harry, for pointing out that memorizing the trigonometric identities is not actually on the syllabus, and to Ewan for pointing out a stupid mistake in the original version of these answers.