If you times 24 by 48 then you get 1,152. 3 times 24 is 72. You would then subtract 1,152 by 72 which equals 1,080 postcards left is what the shop has to sell.
The answer is:
-m+2n-2
Explanation:
11-2m-(n+5-m-3n+8)
11-2m-n-5+m+3n-8
11-5-8-2m+m+3n-n
-2-m+2n
-m+2n-2