Stereoisomers

You may well find this problem difficult, as it requires three dimensional visualisation of two dimensional images. If you cannot manage this in your head, why not try:

-Using a free chemical drawing program, such as 'Chemsketch' to draw and rotate the molecules.
-Using a modelling kit to construct the molecules, so that you can rotate them to see if they can be superimposed.
-Try using your hands! This may prove difficult for molecules with several substituents!!