The question is ambiguous, to the extent that it is not absolutely definite whether the petunias are inside or outside the 3x5 area. In either case the area is the difference between the area-including-petunias and the area excluding petunias. So if it outside, the answer is 5x7 - 3x5, or 4x5 ie 20. Or if it is inside the area it is 3x5 - 1x3 ie 12.