Plsss help me anyone

The answer is 31, because the other lengths have gotten 10 added onto them:

25-10=15, 15+10=25

So this way, it would be 21+10=31, 31-10=21

plants store their excess carbohydrate in the form of starch so that it could be used as energy source later on. the excess carbohydrate is converted into starch in the plastid i.e. chloroplast & this process occurs in the presence of sunlight. it is an insoluble polymer of glucose which gets stored in cell organelles named as amyloplasts. in some plants, starch is also stored in roots and embryos.