Drag each expression into the correct category

Bring the fraction a/a-4 to a denominator of 16-a^2

Carl's candies has determined that a candy bar measuring 3 inches long has a z-score of +1 and a candy bar measuring 3.75 inches long has a z-score of +2. what is the standard deviation of the length of candy bars produced at carl's candies?

Which equation can be used to measure the density of water in a spherical water tank? let w represent the number of water particles within the region.