You need to change the dimensions to yards. There are 3 feet in a yard so divide each of the dimensions by 3. Thus, for the slab, you have
50/3 yards by 25.5/3 yards by 0.5/3 yards.
Thus the volume is
50/3 25.5/3 .05/3 = 23.6 cubic yards.
You didn't tell us the thickness of the walls so can't do those calculations but the method is the same. For each of the 4 walls divide the dimensions in feet by 3 to convert to yards and then multiply the three dimensions to get the volume in cubic yards.