This might be a really simple Geometry rule that I'm missing, but I can't understand how in this problem they automatically knew to extend the truss to point X and knew that it was an additional 2 little triangles. I know that if I were to solve the entire truss using the typical tools like law of sines/cosines, and pythagorus, I could perhaps conclude the same thing, but it seems like there is a big shortcut I am missing to do this instantly.
attached are print screens from Meriam & Kraige Statics textbook of the problem and the solution: