anyone knows css?

web design , basic Css writing 
I wrote the code, but for some reason it's not working.. 

. Here you'll create the receding cube effect that appears in the center of the page. The cube has been constructed by creating a div element with the id cube containing five div elements belonging to the cube_face class with the ids cube_bottom, cube_top, cube_left, cube_right, and cube_front. (There will be no back face for this cube.) Currently the five faces are superimposed upon each other. To create the cube you have to shift and rotate each face in 3D space so that they form the five faces of the cube. First, position the cube on the page by creating a style rule for the div#cube selector containing the following styles: a.  Place the element using relative positioning. b.  Set the top margin to 180 pixels, the bottom margin to 150 pixels, and the left/right margins to auto. c.  Set the width and height to 400 pixels. d.  Set the perspective of the space to 450 pixels. 

11. For each div element of the cube_face class, create a style rule that places the faces with absolute positioning and sets their width and height to 400 pixels. 

Explore 12. Finally, you'll construct the cube by positioning each of the five faces in 3D space so that they form the shape of a cube. Add the following style rules for each of the five faces to transform their appearance. a.  Translate the cube_front div element -50 pixels along the z-axis. b.  Translate the cube_left div element -200 pixels along the x-axis and rotate it 90° around the y-axis. c.  Translate the cube_right div element 200 pixels along the x-axis and rotate it 90° counter-clockwise around the y-axis. d.  Translate the cube_top div element -200 pixels along the y-axis and rotate it 90° counter-clockwise around the x-axis. e.  Translate the cube_bottom div element 200 pixels along the y-axis and rotate it 90° around the x-axis.