[tex]\bigstar{\boxed{\pmb{Area\:of\:a\:Rectangle:\:A=LW}}[/tex]
[tex]\sf{A=8*5}[/tex]
[tex]\sf{A=40~ft^2}[/tex]
[tex]\rule{200}{2}[/tex]
[tex]\sf{A=4*8}[/tex]
[tex]\sf{A=32~ft^2}[/tex]
Now, add the areas:-
[tex]\sf{A=40+32+32~(since~there~are~2~walls~with~this~area)}[/tex]
[tex]\sf{A=72+32}[/tex]
[tex]\sf{A=104}[/tex]
Hope everything is clear; if you need any explanation/clarification, kindly let me know, and I will comment and/or edit my answer :)