Working through the interactivity should help you with this. It is also worth going back to the first page of the interactivity and trying out your ideas there.

There are several possible approaches to the final problem:

1. Using Pythagoras theorem - which is hard work.
2. Using the fact that the square can be split into 45° right angled triangles and some simple trig.
3. Some symmetry and the differences in the x and y coordinates of the vertices and the coordinates of the centre of the square.