It's polka dot to purple, to yield four half-square triangles; polka to background, four HSTs; purple to background, four HSTs, and then four background squares.
Bear in mind that when you sew around all four sides and cut on the diagonals (a very quick, very easy method), the diagonals are on the bias edge of the fabric and can be a little bendy and temperamental when sewing them. People recommend using lots of starch to get those edges to behave. Otherwise, you can make two HSTs at a time and not have to deal with bias edges.
When I make HSTs, I always make them big and then trim them down to size. Yes, it's a big pain, but it makes the blocks so much more consistent and the end result so much better. I've actually come to the conclusion that it's worth the time to trim them down, and I'm all about quick quilting!