There are several different ways of solving this
problem. It is a good example to bring home the point that, to
be an expert problem solver, and to understand a piece of
mathematics, it is not enough to find an answer. One should ask
oneself "Have I used the best method?". This problem can be solved
using trig formulae and the cosine rule but it can be solved more
straightforwardly using only Pythagoras theorem.
Possible approach
Challenge the class to find different methods of solution.