如何把這圖形拼成正方形?

Nice puzzle!

If we notice that there are essentially 8 squares, we would figure out that the side of the final square has a length of 2sqrt(2) units, so the cuts must be at 45 degrees to any of the sides.

1. Make two perpendicular cuts to divide the figure in four pieces.
http://i213.photobucket.com/albums/cc180/31415926535897932384626/puzzle.jpg
2. Flip two of the pieces over and assemble.
http://i213.photobucket.com/albums/cc180/31415926535897932384626/puzzle_3.jpg