Careful attention to detail and a little critical thinking is required for this exercise. First draw a polygon representing the profile of the 2nd floor slab and place it correctly in 3d space. Next define the L-shaped path along Boylston and across Exeter that the extrusion must follow. Use SketchUp's "Follow Me" command to create the initial extrusion, and then leverage SketchUp's "stickiness" to modify the horizontal boundaries of the slab itself.