Answer:
the formula for the width of the photo on the cake
Explanation:
The term 3n is best described as the formula for the width of the photo on the cake. That is because, as described in the question the original photo has a width of length n. While the photo on the cake is 3 times that of n, meaning that the formula is stating that the width of the photo is 3 multiplied by the value of n. Which is represented as 3n, just as shown in the question. Therefore, that would be the best representation of that term.