Find the total surface area of a 3-D composite figure? Do you subtract one base from one of the figures or both of its bases?

What does the shape look like?
Ok, so say you are calculating the surface area of the shape in the image below.
You would calculate the area of both shapes (the vertical box and the horizontal one), then subtract the area of the left face of the left rectangle (it is 10x20). That way, you are subtracting the area from the tall box. Make sure when you calculate the area of the short box that you don’t include the left face (which is non-existent because it is attached to the tall box).