Thread: triangle question

Here is a question(see attachment) a friend of mine preparing for SAT sent me.

What does the triangle signify?I cant make head or tail of the question. Havent encountered this type of problem. And how to solve it?

What about the 'a', '2a', 0 etc marked inside the triangles? Are they angles(I doubt it though)?And why all the triangles are equated to zero?Do the triangles represent a particular number(something like determinants)?

Part of the question seems to be missing (the definition).

Thanks for the assurance!I was perplexed.