This means, the birthday cake was divided into 11 pieces, 9 of them were eaten and 2 pieces were not eaten.
To ensure this we can add the fraction 2/11 (pieces left) and 9/11(pieces eaten). Since this results in 1 i.e. a whole cake, total pieces of the cake were 11
Since it is given that there have been 9 slices eaten, we can assume that whatever amount of slices were cut in total will have 9 reduced from the amount. With the fraction 2/11, we know 11-9=2, which fits perfectly into this description so 2 slices have not been eaten.
