To solve this problem you must add the areas of each rectangle that form the prism;
The area of a rectangle can be calculated with the formula:
There are three pairs of equal rectangles, then you can find the surface area of 3 rectangles and multiply each one by 2, then you obtain that the surface area of the prism is:
The answer is $9.50 because you need to take the total of the popcorn which is $16.50. Then subtract that from $45 and get $28.50. The last step to do is divide $28.50 by three to get your final answer= $9.50