Answer and show steps used to get the answer .

Cake:
12x15x2
(12x2)^2=576
(15x2)^2=900
15x12. =180
Add all the answers up
576+900+180=1656 square inches of icing

Cupcakes:
48cupcakes
Radius: 1.5
Pi(r)^2
3.14159(r)^2
3.14159(1.5)^2
3.14159(2.25)
7.0685 square inches

More icing on the cake

First you have to find the amount on icing on both desserts.
cupcake: find the area of the top using πr(squared) which equals about 14.8
multiply by 48 because that's how many cupcakes you would be getting, therefore equaling 710.4"(squared)
cake: 12x15=180 this is the top. 2x15=30, one of the sides, which you would then multiply by 2 to get the opposite side (60) and then the last sides would be 2x12=24. again you would multiply by 2 (48). note add up all the sides. this is the total of your cake - 288"(squared)

The sheetcake would then have more icing.