The rectangular prism building will require more gallons of paint to completely cover because, compared to the rectangular pyramid, the rectangular prism has a larger surface area, thus it would need more paint to completely cover it.
To find the value of y, you need to isolate the variable. To do this, add 21 to -12. This gives you 9. Now, you're equation is set as 9 = y. This means that the value of y is 9.
