If you need to know the amount of frosting to put on the outside of the brownies you need to know perimeter area volume which one

You would need to find the area of the brownies first and then ice them

You need the area, because area is the space on top of those brownies. If it were perimeter, the frosting would only be put on the edges of the brownies and if it were volume the frosting would be put inside the brownies. :)

Not sure what the question is?