It looks like you're doing good with breaking-down the composite into digestible pieces. So, don't stress too much!
Keep in mind that the area for a 3D shape is Bh, where "B" is the area of a face & "h" is the height.
#5
Shape A is 3X4X9.
First, calculate the area of a face:
9X4= 36
Multiply by height:
36X3= 108
The process is the same with shape B.
3X4X8
Calculate area of face:
8X4= 32
Multiply by height:
32X3= 96
Now you add both areas:
108+96= 204 cubic feet
I think you're doing pretty good, just don't over think things too much. Try #8 with the similar process seen here. Just comment if you get stuck.
Hope this helps!