Answer:
3 gallons of paint
Explanation:
Various data is missing, but I think it can be solved with the following assumptions:
The first thing is to assume that 12 gallons was the exact amount needed for the two rooms.
If so, we can say the following where x is the size of the smallest room:
Room 1: It is 2 * x and needs 3 coats of paint, so there would be 6 * x left
Room 2: is x and uses 2 coats of paint therefore 2 * x
The paint sum would be as follows:
6 * x + 2 * x = 8 * x
8 * x = 12
x = 12/8 = 3/2
So:
Room 1: 6 * 3/2 = 9, 9 gallons of paint needed
Room 2: 2 * 3/2 = 3, 3 gallons of paint needed
Room two is the unpainted one so 3 gallons of paint was returned.