With this sort of geometric problem, all you need to do is look and identify, to narrow down your answer. Particularly, you should pick up on any trends. I'm hoping you noticed that using the other two points, you can see that they are dividing by three from the red shape to get the blue shape sides. And you should also notice that the shape is flipped, so if you picture it in the same position as the other one to match up the sides!
