If you followed the directions correctly, you will find that the cut does not go through the top corner of the bottom square. Remember you have cut from the bottom of the checkerboard to the point between the top left two squares, i.e. you have not gone at a 45º angle!!! Consequently the new rectangle formed, is 7 units wide and 9 [] units high, which when multiplied together gives you the original 64 square units.